Cerebellar ataxia, seizures, premature death, and cardiac abnormalities in mice with targeted disruption of the Cacna2d2 gene

The du(2J) mouse model of ataxia and absence epilepsy has deficient cannabinoid CB₁ receptor-mediated signalling

Cerebellar ataxias are a group of progressive, debilitating diseases often associated with abnormal Purkinje cell (PC) firing and/or degeneration. Many animal models of cerebellar ataxia display abnormalities in Ca²⁺ channel function. The 'ducky' du(2J) mouse model of ataxia and absence epilepsy represents a clean knock-out of the auxiliary Ca²⁺ channel subunit α2δ-2, and has been associated with deficient Ca²⁺ channel function in the cerebellar cortex. Here, we investigate effects of du(2J) mutation on PC layer (PCL) and granule cell layer (GCL) neuronal spiking activity and, also, inhibitory neurotransmission at interneurone-Purkinje cell (IN-PC) synapses. Increased neuronal firing irregularity was seen in the PCL and, to a less marked extent, in the GCL in du(2J)/du(2J), but not +/du(2J), mice; these data suggest that the ataxic phenotype is associated with lack of precision of PC firing, that may also impinge on GC activity and requires expression of two du(2J) alleles to manifest fully. The du(2J) mutation had no clear effect on spontaneous inhibitory postsynaptic current (sIPSC) frequency at IN-PC synapses, but was associated with increased sIPSC amplitudes. du(2J) mutation ablated cannabinoid CB1 receptor (CB1R)-mediated modulation of spontaneous neuronal spike firing and CB1R-mediated presynaptic inhibition of synaptic transmission at IN-PC synapses in both +/du(2J) and du(2J)/du(2J) mutants, effects that occurred in the absence of changes in CB1R expression. These results demonstrate that the du(2J) ataxia model is associated with deficient CB1R signalling in the cerebellar cortex, putatively linked with compromised Ca²⁺ channel activity and the ataxic phenotype.

X-linked dystonia parkinsonism syndrome (XDP, lubag): disease-specific sequence change DSC3 in TAF1/DYT3 affects genes in vesicular transport and dopamine metabolism

X-chromosomal dystonia parkinsonism syndrome (XDP, 'lubag') is associated with sequence changes within the TAF1/DYT3 multiple transcript system. Although most sequence changes are intronic, one, disease-specific single-nucleotide change 3 (DSC3), is located within an exon (d4). Transcribed exon d4 occurs as part of multiple splice variants. These variants include exons d3 and d4 spliced to exons of TAF1, and an independent transcript composed of exons d2-d4. Location of DSC3 in exon d4 and utilization of this exon in multiple splice variants suggest an important role of DSC3 in the XDP pathogenesis. To test this hypothesis, we transfected neuroblastoma cells with four expression constructs, including exons d2-d4 [d2-d4/wild-type (wt) and d2-d4/DSC3] and d3-d4 (d3-d4/wt and d3-d4/DSC3). Expression profiling revealed a dramatic effect of DSC3 on overall gene expression. Three hundred and sixty-two genes differed between cells containing d2-d4/wt and d2-d4/DSC3. Annotation clustering revealed enrichment of genes related to vesicular transport, dopamine metabolism, synapse function, Ca(2+) metabolism and oxidative stress. Two hundred and eleven genes were differentially expressed in d3-d4/wt versus d3-d4/DSC3. Annotation clustering highlighted genes in signal transduction and cell-cell interaction. The data show an important role of physiologically occurring transcript d2-d4 in normal brain function. Interference with this role by DSC3 is a likely pathological mechanism in XDP. Disturbance of dopamine function and of Ca(2+) metabolism can explain abnormal movement; loss of protection against reactive oxygen species may account for the neurodegenerative changes in XDP. Although d3-d4 also affect genes potentially related to neurodegenerative processes, their physiologic role as splice variants of TAF1 awaits further exploration.

Calcium channel auxiliary α2δ and β subunits: trafficking and one step beyond

The voltage-gated calcium channel α(2)δ and β subunits are traditionally considered to be auxiliary subunits that enhance channel trafficking, increase the expression of functional calcium channels at the plasma membrane and influence the channels' biophysical properties. Accumulating evidence indicates that these subunits may also have roles in the nervous system that are not directly linked to calcium channel function. For example, β subunits may act as transcriptional regulators, and certain α(2)δ subunits may function in synaptogenesis. The aim of this Review is to examine both the classic and novel roles for these auxiliary subunits in voltage-gated calcium channel function and beyond.

Mutant PrP suppresses glutamatergic neurotransmission in cerebellar granule neurons by impairing membrane delivery of VGCC α(2)δ-1 Subunit

How mutant prion protein (PrP) leads to neurological dysfunction in genetic prion diseases is unknown. Tg(PG14) mice synthesize a misfolded mutant PrP which is partially retained in the neuronal endoplasmic reticulum (ER). As these mice age, they develop ataxia and massive degeneration of cerebellar granule neurons (CGNs). Here, we report that motor behavioral deficits in Tg(PG14) mice emerge before neurodegeneration and are associated with defective glutamate exocytosis from granule neurons due to impaired calcium dynamics. We found that mutant PrP interacts with the voltage-gated calcium channel α(2)δ-1 subunit, which promotes the anterograde trafficking of the channel. Owing to ER retention of mutant PrP, α(2)δ-1 accumulates intracellularly, impairing delivery of the channel complex to the cell surface. Thus, mutant PrP disrupts cerebellar glutamatergic neurotransmission by reducing the number of functional channels in CGNs. These results link intracellular PrP retention to synaptic dysfunction, indicating new modalities of neurotoxicity and potential therapeutic strategies.

The alpha2delta ligand gabapentin inhibits the Rab11-dependent recycling of the calcium channel subunit alpha2delta-2

The α2δ subunits of voltage-gated calcium channels are important modulatory subunits that enhance calcium currents and may also have other roles in synaptogenesis. The antiepileptic and antiallodynic drug gabapentin (GBP) binds to the α2δ-1 and α2δ-2 isoforms of this protein, and its binding may disrupt the binding of an endogenous ligand, required for their correct function. We have shown previously that GBP produces a chronic inhibitory effect on calcium currents by causing a reduction in the total number of α2δ and α1 subunits at the cell surface. This action of GBP is likely to be attributable to a disruption of the trafficking of α2δ subunits, either to or from the plasma membrane. We studied the effect of GBP on the internalization of, and insertion into the plasma membrane of α2δ-2 using an α-bungarotoxin binding site-tagged α2δ-2 subunit, and a fluorescent derivative of α-bungarotoxin. We found that GBP specifically disrupts the insertion of α2δ-2 from post-Golgi compartments to the plasma membrane, and this effect was prevented by a mutation of α2δ-2 that abolishes its binding to GBP. The coexpression of the GDP-bound Rab11 S25N mutant prevented the GBP-induced decrease in α2δ-2 cell surface levels, both in cell lines and in primary neurons, and the GBP-induced reduction in calcium channel currents. In contrast, the internalization of α2δ-2 was unaffected by GBP. We conclude that GBP acts by preventing the recycling of α2δ-2 from Rab11-positive recycling endosomes to the plasma membrane.

Antiseizure effects of 3alpha-androstanediol and/or 17beta-estradiol may involve actions at estrogen receptor beta

Testosterone (T), the principal androgen secreted by the testes, can have antiseizure effects. Some of these effects may be mediated by T's metabolites. T is metabolized to 3alpha-androstanediol (3alpha-diol). T, but not 3alpha-diol, binds androgen receptor. We investigated effects of 3alpha-diol (1 mg/kg, SC) and/or an androgen receptor blocker (flutamide 10 mg, SC), 1 hour prior to administration of pentylenetetrazol (85 mg/kg, IP). Juvenile male rats administered 3alpha-diol had less seizure activity than those administered vehicle. Flutamide had no effects. T is aromatized to 17beta-estradiol (E(2)), which, like 3alpha-diol, acts at estrogen receptors (ERs). Selective estrogen receptor modulators that favor ERalpha (propyl pyrazole triol, 17alpha-E(2)) or ERbeta (diarylpropionitrile, coumestrol, 3alpha-diol), or both (17beta-E(2)), were administered (0.1 mg/kg, SC) to juvenile male rats 1 hour before pentylenetetrazol. Estrogens with activity at ERbeta, but not those selective for ERalpha, produced antiseizure effects. Actions at ERbeta may underlie some antiseizure effects of T's metabolites.

Seizure susceptibility is associated with altered protein expression of voltage-gated calcium channel subunits in inferior colliculus neurons of the genetically epilepsy-prone rat

The inferior colliculus (IC) is the consensus site for seizure initiation in the genetically epilepsy-prone rat (GEPR). We have previously reported that the current density of high threshold voltage-activated (HVA) calcium (Ca(2+)) channels was markedly enhanced in IC neurons of the GEPR-3 (moderate seizure severity substrain of the GEPR). The present study examines whether subunit protein levels of HVA Ca(2+) channels are altered in IC neurons that exhibit enhanced Ca(2+) current density. Quantification shows that the levels of protein expression of the Ca(2+) channel pore-forming alpha1D (L-type) and alpha1E subunits (R-type) were significantly increased in IC neurons of seizure-naive GEPR-3s (SN-GEPR-3s) compared to control Sprague-Dawley (SD) rats. Significant increases and decreases in the levels of protein expression of Ca(2+) channel regulatory beta3 and alpha2delta subunits occurred in IC neurons of SN-GEPR-3s compared to control SD rats, respectively. No changes occurred in the protein expression of Ca(2+) channel pore-forming alpha1A (P/Q-type), alpha1B (N-type) and alpha1C (L-type) subunits in IC neurons of SN-GEPR-3s compared to control SD rats. A single seizure selectively enhanced protein expression of Ca(2+) channel alpha1A subunits in IC neurons of GEPR-3s. Thus, up-regulation of Ca(2+) channel alpha1D and alpha1E subunits may represent the molecular mechanisms for the enhanced current density of L- and R-type of HVA Ca(2+) channels in IC neurons of the GEPR, and may contribute to the genetic basis of their enhanced seizure susceptibility. The up-regulation of Ca(2+) channel alpha1A subunits induced by seizures may contribute to the increasing IC neuronal excitability that results from repetitive seizures in the GEPR.

Targeted disruption of the voltage-dependent calcium channel alpha2/delta-1-subunit

Cardiac L-type voltage-dependent Ca(2+) channels are heteromultimeric polypeptide complexes of alpha(1)-, alpha(2)/delta-, and beta-subunits. The alpha(2)/delta-1-subunit possesses a stereoselective, high-affinity binding site for gabapentin, widely used to treat epilepsy and postherpetic neuralgic pain as well as sleep disorders. Mutations in alpha(2)/delta-subunits of voltage-dependent Ca(2+) channels have been associated with different diseases, including epilepsy. Multiple heterologous coexpression systems have been used to study the effects of the deletion of the alpha(2)/delta-1-subunit, but attempts at a conventional knockout animal model have been ineffective. We report the development of a viable conventional knockout mouse using a construct targeting exon 2 of alpha(2)/delta-1. While the deletion of the subunit is not lethal, these animals lack high-affinity gabapentin binding sites and demonstrate a significantly decreased basal myocardial contractility and relaxation and a decreased L-type Ca(2+) current peak current amplitude. This is a novel model for studying the function of the alpha(2)/delta-1-subunit and will be of importance in the development of new pharmacological therapies.

Functional properties of the CaV1.2 calcium channel activated by calmodulin in the absence of alpha2delta subunits

Voltage-activated CaV1.2 calcium channels require association of the pore-forming alpha1C subunit with accessory CaVbeta and alpha2delta subunits. Binding of a single calmodulin (CaM) to alpha1C supports Ca2+-dependent inactivation (CDI). The human CaV1.2 channel is silent in the absence of CaVbeta and/or alpha2delta. Recently, we found that coexpression of exogenous CaM (CaMex) supports plasma membrane targeting, gating facilitation and CDI of the channel in the absence of CaVbeta. Here we discovered that CaMex and its Ca2+-insensitive mutant (CaM1234) rendered active alpha1C/CaVbeta channel in the absence of alpha2delta. Coexpression of CaMex with alpha1C and beta2d in calcium-channel-free COS-1 cells recovered gating of the channel and supported CDI. Voltage-dependence of activation was shifted by approximately +40 mV to depolarization potentials. The calcium current reached maximum at +40 mV (20 mM Ca2+) and exhibited approximately 3 times slower activation and 5 times slower inactivation kinetics compared to the wild-type channel. Furthermore, both CaMex and CaM1234 accelerated recovery from inactivation and induced facilitation of the calcium current by strong depolarization prepulse, the properties absent from the human vascular/neuronal CaV1.2 channel. The data suggest a previously unknown action of CaM that in the presence of CaVbeta; translates into activation of the alpha2delta-deficient calcium channel and alteration of its properties.

New molecular targets for antiepileptic drugs: alpha(2)delta, SV2A, and K(v)7/KCNQ/M potassium channels

Many currently prescribed antiepileptic drugs (AEDs) act via voltage-gated sodium channels, through effects on gamma-aminobutyric acid-mediated inhibition, or via voltage-gated calcium channels. Some newer AEDs do not act via these traditional mechanisms. The molecular targets for several of these nontraditional AEDs have been defined using cellular electrophysiology and molecular approaches. Here, we describe three of these targets: alpha(2)delta, auxiliary subunits of voltage-gated calcium channels through which the gabapentinoids gabapentin and pregabalin exert their anticonvulsant and analgesic actions; SV2A, a ubiquitous synaptic vesicle glycoprotein that may prepare vesicles for fusion and serves as the target for levetiracetam and its analog brivaracetam (which is currently in late-stage clinical development); and K(v)7/KCNQ/M potassium channels that mediate the M-current, which acts a brake on repetitive firing and burst generation and serves as the target for the investigational AEDs retigabine and ICA-105665. Functionally, all of the new targets modulate neurotransmitter output at synapses, focusing attention on presynaptic terminals as critical sites of action for AEDs.

Antiseizure effects of 5*-androstane-3*,7beta-diol may be independent of actions at estrogen receptor beta

Testosterone (T), the principal androgen secreted by the testes, can have antiseizure effects; however, the mechanism(s) underlying this action is not well understood. T is metabolized to dihydrotestosterone (DHT) by the enzyme 5*-reductase. DHT is then converted to 5*-androstane-3*,17beta-diol (3*-diol) by the enzyme 3*-hydroxysteroid dehydrogenase. T and DHT bind with high affinity to intracellular androgen receptors; however, 3*-diol does not. The mnemonic effects of 3*-diol are mediated in part through the beta isoform of estrogen receptors (ERbeta) in the hippocampus. As such, we investigated whether 3*-diol has antiseizure effects in mice that require action at ERbeta. 3*-Diol (2 mg/kg subcutaneously) was administered to wild-type C57/B6 mice and heterozygous and homozygous ERbeta knockout (betaERKO) mice 1 hour prior to administration of pentylenetetrazol (PTZ; 85 mg/kg intraperitoneally). Mice administered 3*-diol had significantly longer latencies to clonic seizure and death and lower seizure scores than did mice administered vehicle. This pattern of effects was observed in wild-type or betaERKO mice. Thus, for these mice, the antiseizure effects of 3*-diol for the chemoconvulsant PTZ occur independent of actions at ERbeta.

Evaluation of the 3p21.3 tumour-suppressor gene cluster

Deletions of the 3p21.3 region are a frequent and early event in the formation of lung, breast, kidney and other cancers. Intense investigation of allelic losses and the discovery of overlapping homozygous deletions in lung and breast tumour-cell lines have defined a minimal critical 120 kb deletion region containing eight genes and likely to harbor one or more tumour-suppressor genes (TSGs). The candidate genes are HYAL2, FUS1, Ras-associated factor 1 (RASSF1), BLU/ZMYND10, NPR2L, 101F6, PL6 and CACNA2D2. Recent research indicates that several of these genes can suppress the growth of lung and other tumour cells. Furthermore, some genes (RASSF1A and BLU/ZMYND10) are very frequently inactivated by non-classical mechanisms such as promoter hypermethylation resulting in loss of expression. These data indicate that the 120 kb critical deletion region at 3p21.3 may represent a TSG cluster with preferential inactivation of particular genes depending on tumour type. The eight genes within this region and their potential role in cancer will be the focus of this review.

Autoimmunity, spontaneous tumourigenesis, and IL-15 insufficiency in mice with a targeted disruption of the tumour suppressor gene Fus1

The Fus1 gene resides in the critical 3p21.3 human chromosomal region deleted in lung and breast cancers. Recently, the tumour suppressor properties of Fus1 were confirmed experimentally by intra-tumoural administration of Fus1 that suppressed experimental lung metastasis in mice. We generated Fus1-deficient mice that were viable, fertile, and demonstrated a complex immunological phenotype. Animals with a disrupted Fus1 gene developed signs of autoimmune disease, such as vasculitis, glomerulonephritis, anaemia, circulating autoantibodies, and showed an increased frequency of spontaneous vascular tumours. Preliminary analysis of immune cell populations revealed a consistent defect in NK cell maturation in Fus1 null mice that correlated with changes in the expression of IL-15. Injection of IL-15 into Fus1 knockout mice completely rescued the NK cell maturation defect. Based on these results, we propose the hypothesis that Fus1 deficiency affects NK cell maturation through the reduction of IL-15 production but does not directly alter their developmental capacity. Since acquired immunity was not affected in Fus1-deficient animals, we suggest a relationship between the Fus1 protein and the regulation of innate immunity via IL-15 production. The increased frequency of spontaneous cancers and the development of an autoimmune syndrome in Fus1 null mice imply that these mice could serve as a model for studying molecular mechanisms of anti-tumour immunity and autoimmunity.

Functional biology of the alpha(2)delta subunits of voltage-gated calcium channels

In this review, we examine what is known about the mechanism of action of the auxiliary alpha2delta subunits of voltage-gated Ca(2+) (Ca(v)) channels. First, to provide some background on the alpha2delta proteins, we discuss the genes encoding these channels, in addition to the topology and predicted structure of the alpha2delta subunits. We then describe the effects of alpha2delta subunits on the biophysical properties of Ca(v) channels and their physiological function. All alpha2delta subunits increase the density at the plasma membrane of Ca(2+) channels activated by high voltage, and we discuss what is known about the mechanism underlying this trafficking. Finally, we consider the link between alpha2delta subunits and disease, both in terms of spontaneous and engineered mouse mutants that show cerebellar ataxia and spike-wave epilepsy, and in terms of neuropathic pain and the mechanism of action of the gabapentinoid drugs - small-molecule ligands of the alpha2delta-1 and alpha2delta-2 subunits.

Molecular targets for antiepileptic drug development

This review considers how recent advances in the physiology of ion channels and other potential molecular targets, in conjunction with new information on the genetics of idiopathic epilepsies, can be applied to the search for improved antiepileptic drugs (AEDs). Marketed AEDs predominantly target voltage-gated cation channels (the alpha subunits of voltage-gated Na+ channels and also T-type voltage-gated Ca2+ channels) or influence GABA-mediated inhibition. Recently, alpha2-delta voltage-gated Ca2+ channel subunits and the SV2A synaptic vesicle protein have been recognized as likely targets. Genetic studies of familial idiopathic epilepsies have identified numerous genes associated with diverse epilepsy syndromes, including genes encoding Na+ channels and GABA(A) receptors, which are known AED targets. A strategy based on genes associated with epilepsy in animal models and humans suggests other potential AED targets, including various voltage-gated Ca2+ channel subunits and auxiliary proteins, A- or M-type voltage-gated K+ channels, and ionotropic glutamate receptors. Recent progress in ion channel research brought about by molecular cloning of the channel subunit proteins and studies in epilepsy models suggest additional targets, including G-protein-coupled receptors, such as GABA(B) and metabotropic glutamate receptors; hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel subunits, responsible for hyperpolarization-activated current Ih; connexins, which make up gap junctions; and neurotransmitter transporters, particularly plasma membrane and vesicular transporters for GABA and glutamate. New information from the structural characterization of ion channels, along with better understanding of ion channel function, may allow for more selective targeting. For example, Na+ channels underlying persistent Na+ currents or GABA(A) receptor isoforms responsible for tonic (extrasynaptic) currents represent attractive targets. The growing understanding of the pathophysiology of epilepsy and the structural and functional characterization of the molecular targets provide many opportunities to create improved epilepsy therapies.

Murine cathepsin F deficiency causes neuronal lipofuscinosis and late-onset neurological disease

Cathepsin F (cat F) is a widely expressed lysosomal cysteine protease whose in vivo role is unknown. To address this issue, mice deficient in cat F were generated via homologous recombination. Although cat F-/- mice appeared healthy and reproduced normally, they developed progressive hind leg weakness and decline in motor coordination at 12 to 16 months of age, followed by significant weight loss and death within 6 months. cat F was found to be expressed throughout the central nervous system (CNS). cat F-/- neurons accumulated eosinophilic granules that had features typical of lysosomal lipofuscin by electron microscopy. Large amounts of autofluorescent lipofuscin, characteristic of the neurodegenerative disease neuronal ceroid lipofuscinosis (NCL), accumulated throughout the CNS but not in visceral organs, beginning as early as 6 weeks of age. Pronounced gliosis, an indicator of neuronal stress and neurodegeneration, was also apparent in older cat F-/- mice. cat F is the only cysteine cathepsin whose inactivation alone causes a lysosomal storage defect and progressive neurological features in mice. The late onset suggests that this gene may be a candidate for adult-onset NCL.

New Horizons in the development of antiepileptic drugs: Innovative strategies

The past decades have brought many advances to the treatment of epilepsy. However, despite the continued development and release of new antiepileptic drugs, many patients have seizures that do not respond to drug therapy or have related side effects that preclude continued use. Even in patients in whom pharmacotherapy is efficacious, current antiepileptic drugs do not seem to affect the progression or the underlying natural history of epilepsy. Furthermore, there is currently no drug available which prevents the development of epilepsy, e.g. after head trauma or stroke. Thus, there are at least four important goals for the future: (1) development of better antiepileptic ("anti-ictal") drugs with higher efficacy and tolerability to stop seizures compared to current medications; (2) better understanding of processes leading to epilepsy, thus allowing to create therapies aimed at the prevention of epilepsy in patients at risk; (3) development of disease-modifying therapies, interfering with progression of epilepsy, and (4) improved understanding of neurobiological mechanisms of pharmacoresistance, allowing to develop drugs for reversal or prevention of drug resistance. The third Workshop on New Horizons in the Development of Antiepileptic Drugs explored these four goals for improved epilepsy therapy, with a focus on innovative strategies in the search for better anti-ictal drugs, for novel drugs for prevention of epilepsy or its progression, and for drugs overcoming drug resistance in epilepsy. In this conference review, the current status of antiepileptic therapies under development is critically assessed, and innovative approaches for future therapies are highlighted.

The L-type calcium channel in the heart: the beat goes on

Sydney Ringer would be overwhelmed today by the implications of his simple experiment performed over 120 years ago showing that the heart would not beat in the absence of Ca2+. Fascination with the role of Ca2+ has proliferated into all aspects of our understanding of normal cardiac function and the progression of heart disease, including induction of cardiac hypertrophy, heart failure, and sudden death. This review examines the role of Ca2+ and the L-type voltage-dependent Ca2+ channels in cardiac disease.

Norfluoxetine and fluoxetine have similar anticonvulsant and Ca2+ channel blocking potencies

Norfluoxetine is the most important active metabolite of the widely used antidepressant fluoxetine but little is known about its pharmacological actions. In this study the anticonvulsant actions of norfluoxetine and fluoxetine were studied and compared to those of phenytoin and clonazepam in pentylenetetrazol-induced mouse epilepsy models. Pretreatment with fluoxetine or norfluoxetine (20mg/kg s.c.), as well as phenytoin (30 mg/kg s.c.) and clonazepam (0.1mg/kg s.c.) significantly increased both the rate and duration of survival, demonstrating a significant protective effect against pentylenetetrazol-induced epilepsy. These effects of norfluoxetine were similar to those of fluoxetine. According to the calculated combined protection scores, both norfluoxetine and fluoxetine were effective from the concentration of 10mg/kg, while the highest protective action was observed with clonazepam. Effects of norfluoxetine and fluoxetine on voltage-gated Ca2+ channels were evaluated by measuring peak Ba2+ current flowing through the Ca2+ channels upon depolarization using whole cell voltage clamp in enzymatically isolated rat cochlear neurons. The current was reduced equally in a concentration-dependent manner by norfluoxetine (EC50=20.4+/-2.7 microM, Hill coefficient=0.86+/-0.1) and fluoxetine (EC50=22.3+/-3.6 microM, Hill coefficient=0.87+/-0.1). It was concluded that the efficacy of the two compounds in neuronal tissues was equal, either in preventing seizure activity or in blocking the neuronal Ca2+ channels.