Whole-cell and single-channel analysis of P-type calcium currents in cerebellar Purkinje cells of leaner mutant mice

Physiology of Dystonia: Animal Studies

Dystonia is currently ranked as the third most prevalent motor disorder. It is typically characterized by involuntary muscle over- or co-contractions that can cause painful abnormal postures and jerky movements. Dystonia is a heterogenous disorder-across patients, dystonic symptoms vary in their severity, body distribution, temporal pattern, onset, and progression. There are also a growing number of genes that are associated with hereditary dystonia. In addition, multiple brain regions are associated with dystonic symptoms in both genetic and sporadic forms of the disease. The heterogeneity of dystonia has made it difficult to fully understand its underlying pathophysiology. However, the use of animal models has been used to uncover the complex circuit mechanisms that lead to dystonic behaviors. Here, we summarize findings from animal models harboring mutations in dystonia-associated genes and phenotypic animal models with overt dystonic motor signs resulting from spontaneous mutations, neural circuit perturbations, or pharmacological manipulations. Taken together, an emerging picture depicts dystonia as a result of brain-wide network dysfunction driven by basal ganglia and cerebellar dysfunction. In the basal ganglia, changes in dopaminergic, serotonergic, noradrenergic, and cholinergic signaling are found across different animal models. In the cerebellum, abnormal burst firing activity is observed in multiple dystonia models. We are now beginning to unveil the extent to which these structures mechanistically interact with each other. Such mechanisms inspire the use of pre-clinical animal models that will be used to design new therapies including drug treatments and brain stimulation.

The CACNA1A Mutant Disrupts Lysosome Calcium Homeostasis in Cerebellar Neurons and the Resulting Endo-Lysosomal Fusion Defect Can be Improved by Calcium Modulation

Mutations in P/Q type voltage gated calcium channel (VGCC) lead severe human neurological diseases such as episodic ataxia 2, familial hemiplegic migraine 1, absence epilepsy, progressive ataxia and spinocerebellar ataxia 6. The pathogenesis of these diseases remains unclear. Mice with spontaneous mutation in the Cacna1a gene encoding the pore-forming subunit of P/Q type VGCC also exhibit ataxia, epilepsy and neurodegeneration. Based on the previous work showing that the P/Q type VGCC in neurons regulates lysosomal fusion through its calcium channel activity on lysosomes, we utilized CACNA1A mutant mice to further investigate the mechanism by which P/Q-type VGCCs regulate lysosomal function and neuronal homeostasis. We found CACNA1A mutant neurons have reduced lysosomal calcium storage without changing the resting calcium concentration in cytoplasm and the acidification of lysosomes. Immunohistochemistry and transmission electron microscopy reveal axonal degeneration due to lysosome dysfunction in the CACNA1A mutant cerebella. The calcium modulating drug thapsigargin, by depleting the ER calcium store, which locally increases the calcium concentration can alleviate the defective lysosomal fusion in mutant neurons. We propose a model that in cerebellar neurons, P/Q-type VGCC maintains the integrity of the nervous system by regulating lysosomal calcium homeostasis to affect lysosomal fusion, which in turn regulates multiple important cellular processes such as autophagy and endocytosis. This study helps us to better understand the pathogenesis of P/Q-type VGCC related neurodegenerative diseases and provides a feasible direction for future pharmacological treatment.

Losing the Beat: Contribution of Purkinje Cell Firing Dysfunction to Disease, and Its Reversal

The cerebellum is a brain structure that is highly interconnected with other brain regions. There are many contributing factors to cerebellar-related brain disease, such as altered afferent input, local connectivity, and/or cerebellar output. Purkinje cells (PC) are the principle cells of the cerebellar cortex, and fire intrinsically; that is, they fire spontaneous action potentials at high frequencies. This review paper focuses on PC intrinsic firing activity, which is altered in multiple neurological diseases, including ataxia, Huntington Disease (HD) and autism spectrum disorder (ASD). Notably, there are several cases where interventions that restore or rescue PC intrinsic activity also improve impaired behavior in these mouse models of disease. These findings suggest that rescuing PC firing deficits themselves may be sufficient to improve impairment in cerebellar-related behavior in disease. We propose that restoring PC intrinsic firing represents a good target for drug development that might be of therapeutic use for several disorders.

Ion channel dysfunction in cerebellar ataxia

Cerebellar ataxias constitute a heterogeneous group of disorders that result in impaired speech, uncoordinated limb movements, and impaired balance, often ultimately resulting in wheelchair confinement. Motor dysfunction in ataxia can be attributed to dysfunction and degeneration of neurons in the cerebellum and its associated pathways. Recent work has suggested the importance of cerebellar neuronal dysfunction resulting from mutations in specific ion-channels that regulate membrane excitability in the pathogenesis of cerebellar ataxia in humans. Importantly, even in ataxias not directly due to ion-channel mutations, transcriptional changes resulting in ion-channel dysfunction are tied to motor dysfunction and degeneration in models of disease. In this review, we describe the role that ion-channel dysfunction plays in a variety of cerebellar ataxias, and postulate that a potential therapeutic strategy that targets specific ion-channels exists for cerebellar ataxia.

The first knockin mouse model of episodic ataxia type 2

Episodic ataxia type 2 (EA2) is an autosomal dominant disorder associated with attacks of ataxia that are typically precipitated by stress, ethanol, caffeine or exercise. EA2 is caused by loss-of-function mutations in the CACNA1A gene, which encodes the α1A subunit of the CaV2.1 voltage-gated Ca(2+) channel. To better understand the pathomechanisms of this disorder in vivo, we created the first genetic animal model of EA2 by engineering a mouse line carrying the EA2-causing c.4486T>G (p.F1406C) missense mutation in the orthologous mouse Cacna1a gene. Mice homozygous for the mutated allele exhibit a ~70% reduction in CaV2.1 current density in Purkinje cells, though surprisingly do not exhibit an overt motor phenotype. Mice hemizygous for the knockin allele (EA2/- mice) did exhibit motor dysfunction measurable by rotarod and pole test. Studies using Cre-flox conditional genetics explored the role of cerebellar Purkinje cells or cerebellar granule cells in the poor motor performance of EA2/- mice and demonstrate that manipulation of either cell type alone did not cause poor motor performance. Thus, it is possible that subtle dysfunction arising from multiple cell types is necessary for the expression of certain ataxia syndromes.

Cdk5/p35 is required for motor coordination and cerebellar plasticity

Previous studies have implicated the role of Purkinje cells in motor learning and the underlying mechanisms have also been identified in great detail during the last decades. Here we report that cyclin-dependent kinase 5 (Cdk5)/p35 in Purkinje cell also contributes to synaptic plasticity. We previously showed that p35(-/-) (p35 KO) mice exhibited a subtle abnormality in brain structure and impaired spatial learning and memory. Further behavioral analysis showed that p35 KO mice had a motor coordination defect, suggesting that p35, one of the activators of Cdk5, together with Cdk5 may play an important role in cerebellar motor learning. Therefore, we created Purkinje cell-specific conditional Cdk5/p35 knockout (L7-p35 cKO) mice, analyzed the cerebellar histology and Purkinje cell morphology of these mice, evaluated their performance with balance beam and rota-rod test, and performed electrophysiological recordings to assess long-term synaptic plasticity. Our analyses showed that Purkinje cell-specific deletion of Cdk5/p35 resulted in no changes in Purkinje cell morphology but severely impaired motor coordination. Furthermore, disrupted cerebellar long-term synaptic plasticity was observed at the parallel fiber-Purkinje cell synapse in L7-p35 cKO mice. These results indicate that Cdk5/p35 is required for motor learning and involved in long-term synaptic plasticity.

Voltage-gated Ca2+ channels in accessory lobe neurons of the chick

Birds have ten pairs of protrusions, "accessory lobes", on the lateral sides of the lumbosacral spinal cord. It has been proposed that accessory lobes act as a sensory organ of equilibrium and neurons in accessory lobes transmit sensory information to the motor center. We have reported that cells in chick accessory lobes express functional voltage-gated Na(+) and K(+) channels and generate action potentials. In this study, we examined properties of voltage-gated Ca(2+) channels (VGCCs). The amplitude of voltage-gated Ca(2+) channel currents carried by Ca(2+) and Ba(2+) increased gradually during 10 min rather than showing the usual run-down. The current-voltage relationship of Ba(2+) currents was consistent with that of the high-voltage-activated Ca(2+) channel. The proportion of Ba(2+) currents inhibited by ω-conotoxin GVIA was larger than 80%, indicating that the major subtype is N type. Amplitudes of tail currents of Ca(2+) currents evoked by repetitive pulses at 50 Hz are stable for 1 s. If the major subtype of VGCCs at synaptic terminals is also N type, this property may contribute to the establishment of stable synaptic connections between accessory lobe neurons, which are reported to fire at frequencies higher than 15 Hz, and postsynaptic neurons in the spinal cord.

Enhanced synaptic inhibition disrupts the efferent code of cerebellar Purkinje neurons in leaner Cav2.1 Ca 2+ channel mutant mice

Cerebellar Purkinje cells (PCs) encode afferent information in the rate and temporal structure of their spike trains. Both spontaneous firing in these neurons and its modulation by synaptic inputs depend on Ca(2+) current carried by Ca(v)2.1 (P/Q) type channels. Previous studies have described how loss-of-function Ca(v)2.1 mutations affect intrinsic excitability and excitatory transmission in PCs. This study examines the effects of the leaner mutation on fast GABAergic transmission and its modulation of spontaneous firing in PCs. The leaner mutation enhances spontaneous synaptic inhibition of PCs, leading to transitory reductions in PC firing rate and increased spike rate variability. Enhanced inhibition is paralleled by an increase in the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) measured under voltage clamp. These differences are abolished by tetrodotoxin, implicating effects of the mutation on spike-induced GABA release. Elevated sIPSC frequency in leaner PCs is not accompanied by increased mean firing rate in molecular layer interneurons, but IPSCs evoked in PCs by direct stimulation of these neurons exhibit larger amplitude, slower decay rate, and a higher burst probability compared to wild-type PCs. Ca(2+) release from internal stores appears to be required for enhanced inhibition since differences in sIPSC frequency and amplitude in leaner and wild-type PCs are abolished by thapsigargin, an ER Ca(2+) pump inhibitor. These findings represent the first account of the functional consequences of a loss-of-function P/Q channel mutation on PC firing properties through altered GABAergic transmission. Gain in synaptic inhibition shown here would compromise the fidelity of information coding in these neurons and may contribute to impaired cerebellar function resulting from loss-of function mutations in the Ca(V)2.1 channel gene.

Channelopathies in Cav1.1, Cav1.3, and Cav1.4 voltage-gated L-type Ca2+ channels

Voltage-gated Ca2+ channels couple membrane depolarization to Ca2+-dependent intracellular signaling events. This is achieved by mediating Ca2+ ion influx or by direct conformational coupling to intracellular Ca2+ release channels. The family of Cav1 channels, also termed L-type Ca2+ channels (LTCCs), is uniquely sensitive to organic Ca2+ channel blockers and expressed in many electrically excitable tissues. In this review, we summarize the role of LTCCs for human diseases caused by genetic Ca2+ channel defects (channelopathies). LTCC dysfunction can result from structural aberrations within their pore-forming alpha1 subunits causing hypokalemic periodic paralysis and malignant hyperthermia sensitivity (Cav1.1 alpha1), incomplete congenital stationary night blindness (CSNB2; Cav1.4 alpha1), and Timothy syndrome (Cav1.2 alpha1; reviewed separately in this issue). Cav1.3 alpha1 mutations have not been reported yet in humans, but channel loss of function would likely affect sinoatrial node function and hearing. Studies in mice revealed that LTCCs indirectly also contribute to neurological symptoms in Ca2+ channelopathies affecting non-LTCCs, such as Cav2.1 alpha1 in tottering mice. Ca2+ channelopathies provide exciting disease-related molecular detail that led to important novel insight not only into disease pathophysiology but also to mechanisms of channel function.

Genetics of headaches

Insight into the molecular mechanisms involved in primary headaches is important to identify drug targets for improving treatment of patients, but essentially lacking. Genetic research is increasingly successful in pinpointing these mechanisms. Most progress has been made for Familial Hemiplegic Migraine, a rare subtype of migraine with aura. Three genes (CACNA1A, ATP1A2 and SCN1A) have been identified that all encode ion transporters. Cellular and transgenic mouse studies suggest that neuronal hyperexcitability and increased susceptibility to cortical spreading depression, the correlate of migraine aura, are important molecular mechanisms in migraine. Investigating monogenic diseases in which migraine is a prominent feature such as CADASIL, which is caused by mutations in the NOTCH3 gene, can help understanding the pathology of migraine. Candidate gene association studies and linkage studies in the common forms of migraine were less successful. Except for the MTHFR gene no gene variant has been identified yet. Convincingly demonstrated genetic findings in other primary headaches such as cluster headache and tension-type headache are even rarer. However, with current technical possibilities of massive genotyping and international efforts to collect large well-phenotyped patient cohorts, the first gene variants for various primary headache types are likely to be discovered in the coming decade.

Alterations in intracellular calcium ion concentrations in cerebellar granule cells of the CACNA1A mutant mouse, leaner, during postnatal development

Maintaining calcium ion (Ca²+) homeostasis is crucial for normal neuronal function. Altered Ca²+ homeostasis interferes with Ca²+ signaling processes and affects neuronal survival. In this study, we used homozygous leaner and tottering mutant mice, which carry autosomal recessive mutations in the gene coding for the α(1A) pore forming subunit of Ca(V)2.1 (P/Q-type) voltage-gated calcium channels (VGCC). Leaner mice show severe ataxia and epilepsy, while tottering mice are less severely affected. Leaner cerebellar granule cells (CGC) show extensive apoptotic cell death that peaks at postnatal (P) day 20 and continues into adulthood. Intracellular Ca²+ ([Ca²+](i)) concentrations in leaner and tottering mouse Purkinje cells have been described, but [Ca²+](i) concentrations have not been reported for granule cells, the largest neuronal population of the cerebellum. Using the ratiometric dye, Fura-2 AM, we investigated the role of Ca²+ homeostasis in CGC death during postnatal development by demonstrating basal [Ca²+](i), depolarization induced Ca²+ transients, and Ca²+ transients after completely blocking Ca(V)2.1 VGCC. From P20 onward, basal [Ca²+](i) levels in leaner CGC were significantly lower compared to age-matched wild-type CGC. We also compared basal [Ca²+](i) levels in leaner and wild-type CGC to basal [Ca²+](i) in tottering CGC. Potassium chloride induced depolarization revealed no significant difference in Ca²+ transients between leaner and wild-type CGC, indicating that even though leaner CGC have dysfunctional P/Q-type VGCC, Ca²+ transients after depolarization are the same. This suggests that other VGCC are compensating for the dysfunctional P/Q channels. This finding was further confirmed by completely blocking Ca(V)2.1 VGCC using ω-Agatoxin IV-A.

Knockdown of Cav2.1 calcium channels is sufficient to induce neurological disorders observed in natural occurring Cacna1a mutants in mice

The CACNA1A gene encodes the poreforming, voltage-sensitive subunit of the voltage-dependent Ca(v)2.1 calcium channel. Mutations in this gene have been linked to several human disorders, including familial hemiplegic migraine type 1, episodic ataxia type 2, and spinocerebellar ataxia type 6. In mice, mutations of the homolog Cacna1a cause recessively inherited phenotypes in tottering, rolling Nagoya, rocker, and leaner mice. Here we describe two knockdown mice with 28.4+/-3.4% and 13.8+/-3.3% of the wild-type Ca(v)2.1 quantity. 28.4+/-3.4% level mutants displayed ataxia, absence-like seizures and progressive cerebellar atrophy, although they had a normal life span. Mutants with 13.8+/-3.3% level exhibited ataxia severer than the 28.4+/-3.4% level mutants, absence-like seizures and additionally paroxysmal dyskinesia, and died premature around 3 weeks of age. These results indicate that knock down of Ca(v)2.1 quantity to 13.8+/-3.3% of the wild-type level are sufficient to induce the all neurological disorders observed in natural occurring Cacna1a mutants. These knockdown animals with Ca(v)2.1 calcium channels intact can contribute to functional studies of the molecule in the disease.

The ataxic Cacna1a-mutant mouse rolling nagoya: an overview of neuromorphological and electrophysiological findings

Homozygous rolling Nagoya natural mutant mice display a severe ataxic gait and frequently roll over to their side or back. The causative mutation resides in the Cacna1a gene, encoding the pore-forming α₁ subunit of Ca_v2.1 type voltage-gated Ca²⁺ channels. These channels are crucially involved in neuronal Ca²⁺ signaling and in neurotransmitter release at many central synapses and, in the periphery, at the neuromuscular junction. We here review the behavioral, histological, biochemical, and neurophysiological studies on this mouse mutant and discuss its usefulness as a model of human neurological diseases associated with Ca_v2.1 dysfunction.

Hypoalgesic behaviors of P/Q-type voltage-gated Ca2+ channel mutant mouse, rolling mouse Nagoya

Rolling mouse Nagoya (tg(rol)) is a spontaneously occurring P/Q-type voltage-gated Ca2+ channel (VGCC) mutant mouse. A P/Q-type VGCC with the tg(rol) mutation has lower voltage sensitivity of activation, and mice with a homozygous genotype (tg(rol)/tg(rol)) but not with a heterozygous genotype (tg(rol)/+) show impaired motor coordination of the hind limbs. To investigate the roles of P/Q-type VGCC in pain sensing mechanisms, behavioral responses of adult tg(rol) mice to thermal, mechanical and chemical nociceptive stimuli were examined by the plantar, tail-flick, von Frey and formalin tests. The latency of the withdrawal response to thermal stimuli in the plantar or tail-flick tests was significantly longer in tg(rol)/tg(rol) mice than in tg(rol)/+ and wild-type (+/+) mice, and in tg(rol)/+ mice than in +/+ mice. The withdrawal response to mechanical stimuli in the von Frey test was lower in tg(rol)/tg(rol) mice than in +/+ mice. Although the licking time during the first 5 min after the formalin injection was similar among all of the three genotypes, that during 5-60 min was significantly shorter in tg(rol)/tg(rol) mice than in tg(rol)/+ and +/+ mice, and in tg(rol)/+ mice than in +/+ mice. Artificial inflammation induced by injection of complete Freund's adjuvant (CFA) into a hind paw significantly enhanced the withdrawal response recorded in the plantar and von Frey tests regardless of the mouse genotype. The CFA-enhanced response in the tg(rol)/tg(rol) mice was similar to the response in +/+ mice without the CFA injection. These results suggest that tg(rol) mutant mice show hypoalgesic responses caused by a lower sensitivity to nociceptive thermal, mechanical and chemical stimuli. It is concluded that the P/Q-type VGCC has a pro-nociceptive role and that the tg(rol) mutant mouse may be a useful tool to investigate the role of the P/Q-type VGCC in pain sensing mechanisms.

Pogo: a novel spontaneous ataxic mutant mouse

The Pogo (pogo/pogo) mouse is a naturally occurring neurological mutant from a Korean wild-type mouse characterized by loss of balance and motor coordination due to dysfunction of the cerebellum. The Pogo mutation is believed to be an allele of P/Q-type calcium channel mutants such as tottering, leaner, and rolling mouse Nagoya. These mutants have been served as mouse models for a group of neurodegenerative diseases. The overall aim of this minireview is to summarize our current understanding of the ataxic Pogo mouse. To address this issue, we first describe the discovery of Pogo mouse and its morphological and behavioral defects. Then, we focus on the abnormal expression of several molecules in the Pogo cerebellum, including tyrosine hydroxylase, glutamate, corticotrophin-releasing factor, and 5-hydroxytryptamine. Much of this review is concerned with the functional implications of these ectopic molecules in the Pogo cerebellum.

Spinocerebellar ataxia type 6 knockin mice develop a progressive neuronal dysfunction with age-dependent accumulation of mutant CaV2.1 channels

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disorder caused by CAG repeat expansions within the voltage-gated calcium (Ca(V)) 2.1 channel gene. It remains controversial whether the mutation exerts neurotoxicity by changing the function of Ca(V)2.1 channel or through a gain-of-function mechanism associated with accumulation of the expanded polyglutamine protein. We generated three strains of knockin (KI) mice carrying normal, expanded, or hyperexpanded CAG repeat tracts in the Cacna1a locus. The mice expressing hyperexpanded polyglutamine (Sca6(84Q)) developed progressive motor impairment and aggregation of mutant Ca(V)2.1 channels. Electrophysiological analysis of cerebellar Purkinje cells revealed similar Ca(2+) channel current density among the three KI models. Neither voltage sensitivity of activation nor inactivation was altered in the Sca6(84Q) neurons, suggesting that expanded CAG repeat per se does not affect the intrinsic electrophysiological properties of the channels. The pathogenesis of SCA6 is apparently linked to an age-dependent process accompanied by accumulation of mutant Ca(V)2.1 channels.

Impact of the leaner P/Q-type Ca2+ channel mutation on excitatory synaptic transmission in cerebellar Purkinje cells

Loss-of-function mutations in the gene encoding P/Q-type Ca(2+) channels cause cerebellar ataxia in mice and humans, but the underlying mechanism(s) are unknown. These Ca(2+) channels play important roles in regulating both synaptic transmission and intrinsic membrane properties, and defects in either could contribute to ataxia. Our previous work described changes in intrinsic properties and excitability of cerebellar Purkinje cells (PCs) resulting from the leaner mutation, which is known to reduce whole-cell Ca(2+) currents in PCs and cause severe ataxia. Here we describe the impact of this mutation on excitatory synaptic transmission from parallel and climbing fibres (PFs, CFs) to PCs in acute cerebellar slices. We found that in leaner PCs, PF-evoked excitatory postsynaptic currents (PF-EPSCs) are approximately 50% smaller, and CF-evoked EPSCs are approximately 80% larger, than in wild-type (WT) mice. To investigate whether reduced presynaptic Ca(2+) entry plays a role in attenuating PF-EPSCs in leaner mice, we examined paired-pulse facilitation (PPF). We found that PPF is enhanced in leaner, suggesting that reduced presynaptic Ca(2+) entry reduces neurotransmitter release at these synapses. Short-term plasticity was unchanged at CF-PC synapses, suggesting that CF-EPSCs are larger in leaner PCs because of increased synapse number or postsynaptic sensitivity, rather than enhanced presynaptic Ca(2+) entry. To investigate the functional impact of the observed EPSC changes, we also compared excitatory postsynaptic potentials (EPSPs) elicited by PF and CF stimulation in WT and leaner PCs. Importantly, we found that despite pronounced changes in PF- and CF-EPSCs, evoked EPSPs in leaner mice are very similar to those observed in WT animals. These results suggest that changes in synaptic currents and intrinsic properties of PCs produced by the leaner mutation together maintain PC responsiveness to excitatory synaptic inputs. They also implicate other consequences of the leaner mutation as causes of abnormal cerebellar motor control in mutant mice.

Vesicular apparatus, including functional calcium channels, are present in developing rodent optic nerve axons and are required for normal node of Ranvier formation

P/Q-type calcium channels are known to form clusters at the presynaptic membrane where they mediate calcium influx, triggering vesicle fusion. We now report functional P/Q channel clusters in the axolemma of developing central axons that are also associated with sites of vesicle fusion. These channels were activated by axonal action potentials and the resulting calcium influx is well suited to mediate formation of a synaptic style SNARE complex involving SNAP-25, that we show to be located on the axolemma. Vesicular elements within axons were found to be the sole repository of vesicular glutamate in developing white matter. The axonal vesicular elements expressed the glutamate transporter V-ATPase, which is responsible for vesicular glutamate loading. The P/Q channel alpha(1A) subunit was found to be present within the axolemma at early nodes of Ranvier and deleterious mutations of the alpha(1A) subunit, or an associated alpha(2)delta-2 subunit, disrupted the localization of nodal proteins such as voltage-gated sodium channels, beta IV spectrin and CASPR-1. This was associated with the presence of malformed nodes of Ranvier characterized by an accumulation of axoplasmic vesicles under the nodal membrane. The data are consistent with the presence of a vesicular signalling pathway between axons and glial cells that is essential for proper development of the node of Ranvier.

Two novel alleles of tottering with distinct Ca(v)2.1 calcium channel neuropathologies

The calcium channel CACNA1A gene encodes the pore-forming, voltage-sensitive subunit of the voltage-dependent calcium Ca(v)2.1 type channel. Mutations in this gene have been linked to several human disorders, including familial hemiplegic migraine, episodic ataxia 2 and spinocerebellar ataxia type 6. The mouse homologue, Cacna1a, is associated with the tottering, Cacna1a(tg), mutant series. Here we describe two new missense mutant alleles, Cacna1a(tg-4J) and Cacna1a(Tg-5J). The Cacna1a(tg-4J) mutation is a valine to alanine mutation at amino acid 581, in segment S5 of domain II. The recessive Cacna1a(tg-4J) mutant exhibited the ataxia, paroxysmal dyskinesia and absence seizures reminiscent of the original tottering mouse. The Cacna1a(tg-4J) mutant also showed altered activation and inactivation kinetics of the Ca(v)2.1 channel, not previously reported for other tottering alleles. The semi-dominant Cacna1a(Tg-5J) mutation changed a conserved arginine residue to glutamine at amino acid 1252 within segment S4 of domain III. The heterozygous mouse was ataxic and homozygotes rarely survived. The Cacna1a(Tg-5J) mutation caused a shift in both voltage activation and inactivation to lower voltages, showing that this arginine residue is critical for sensing Ca(v)2.1 voltage changes. These two tottering mouse models illustrate how novel allelic variants can contribute to functional studies of the Ca(v)2.1 calcium channel.

Analysis of calcium ion homeostasis and mitochondrial function in cerebellar granule cells of adult CaV 2.1 calcium ion channel mutant mice

CaV 2.1 voltage-gated calcium channels (VGCC) are highly expressed by cerebellar neurons, and their dysfunction is linked to human disorders including familial hemiplegic migraine, episodic ataxia type 2 and spinocerebellar ataxia type 6. Altered calcium homeostasis, due to dysfunctional Ca(V 2.1 VGCC can severely affect mitochondrial function, eventually leading to neuronal cell death. We study leaner and tottering mice, which carry autosomal recessive mutations in the gene coding for the alpha 1A pore-forming subunit of CaV 2.1 VGCC. Both leaner and tottering mice exhibit cerebellar ataxia and epilepsy. Excessive leaner cerebellar granule cell (CGC) death starts soon after postnatal day 10, but it is not known whether the degree of CGC cell death observed in adult leaner mice is significantly different from wild type mice. We used Fluoro-Jade and TUNEL staining to quantify apoptotic cell death in leaner and wild type CGC. We investigated calcium homeostasis, mitochondrial function and generation of reactive oxygen species (ROS) in isolated CGC, using indicator dyes Fura-2AM, TMRM and CMH2DCFDA, respectively. We observed a small but significant increase in number of apoptotic adult leaner CGC. Calcium homeostasis and mitochondrial function also were altered in leaner CGC. However, no significant differences in ROS levels were observed. It is possible that CGC death in leaner mice may be related to mitochondrial dysfunction but may not be directly related to decreased basal intracellular calcium.

The leaner P/Q-type calcium channel mutation renders cerebellar Purkinje neurons hyper-excitable and eliminates Ca2+-Na+ spike bursts

The leaner mouse mutation of the Cacna1a gene leads to a reduction in P-type Ca2+ current, the dominant Ca2+ current in Purkinje cells (PCs). Here, we compare the electro-responsiveness and structure of PCs from age-matched leaner and wild-type (WT) mice in pharmacological isolation from synaptic inputs in cerebellar slices. We report that compared with WT, leaner PCs exhibit lower current threshold for Na+ spike firing, larger subthreshold membrane depolarization, rapid adaptation followed by complete block of Na+ spikes upon strong depolarization, and fail to generate Ca2+-Na+ spike bursts. The Na+ spike waveforms in leaner PCs have slower kinetics, reduced spike amplitude and afterhyperpolarization. We show that a deficit in the P-type Ca2+ current caused by the leaner mutation accounts for most but not all of the changes in mutant PC electro-responsiveness. The selective P-type Ca2+ channel blocker, omega-agatoxin-IVA, eliminated differences in subthreshold membrane depolarization, adaptation of Na+ spikes upon strong current-pulse stimuli, Na+ spike waveforms and Ca2+-Na+ burst activity. In contrast, a lower current threshold for eliciting repetitive Na+ spikes in leaner PCs was still observed after blockade of the P-type Ca2+ current, suggesting secondary effects of the mutation that render PCs hyper-excitable. Higher input resistance, reduced whole-cell capacitance and smaller dendritic size accompanied the enhanced excitability in leaner PCs, indicative of developmental retardation in these cells caused by P/Q-type Ca2+ channel malfunction. Our data indicate that a deficit in P-type Ca2+ current leads to complex functional and structural changes in PCs, impairing their intrinsic and integrative properties.

Increased Ca2+ channel currents in cerebellar Purkinje cells of the ataxic groggy rat

The ataxic groggy rat (strain name; GRY) is an autosomal recessive neurological mutant found in a closed colony of Slc:Wistar rats. Recent genetic analysis has identified the missense (M251K) mutation in the alpha(1) subunit of the Ca(V)2.1 (P/Q-type) voltage-dependent Ca(2+) channel gene (Cacna1a) of GRY rat. In this study, we found that high-voltage-activated (HVA) Ca(2+) channel currents in acutely dissociated Purkinje cells of GRY rats showed increased (not decreased) current density and depolarizing shift of the activation and inactivation curves compared with those of normal Wistar rats. In contrast low-voltage-activated (LVA) Ca(2+) channel currents of GRY rats showed no significant changes. These results suggest that functional alteration of Ca(2+) channel currents in cerebellar Purkinje cells of GRY rats is attributed to the change of HVA Ca(2+) channel currents, and that increased HVA Ca(2+) channel function underlies the cerebellar dysfunction and ataxic phenotype of GRY rats.

Altered functional expression of Purkinje cell calcium channels precedes motor dysfunction in tottering mice

In tottering mice, a point mutation in the gene encoding P-type (Ca(v)2.1) voltage-gated calcium channels results in ataxia, absence epilepsy, and motor dystonia that appear 3-4 weeks postnatally. The aberrant motor behaviors have been linked to cerebellar dysfunction, and adult Purkinje cells (PCs) of tottering mice exhibit calcium-dependent changes in gene transcription suggestive of altered calcium homeostasis. In an attempt to identify early postnatal events important for the development of the behavioral phenotype, we examined calcium channel expression in cerebellar PCs from postnatal days 6-15 (P6-15). Whole cell recording was combined with selective calcium channel antagonists to allow discrimination of the various voltage-activated calcium channels types; early age-dependent differences between tottering and wild-type PCs were found. Wild-type PCs experienced a steady increase in P current density over this period, resulting in a twofold change by P15. In tottering, by contrast, P current density remained unchanged from P6-8 and was only 25% of the wild-type level by P8. A developmental delay in functional expression was implicated in this early deficit, since ensuing gains over the subsequent week brought tottering P current density close to the wild-type level by P15. At this age, tottering PCs also exhibited a 2.2-fold higher L-type calcium current density than that expressed by wild-type PCs. Increases in N current were apparent at some ages, most strikingly within a subset of tottering PCs at P15. Functional R- and T-type calcium current densities were equivalent to wild-type levels at all ages. We conclude that the tottering mutation brings about selective changes in functional calcium channel expression 1 to 2 weeks prior to the appearance of the behavioral deficits, raising the possibility that they represent an early, primary event along the path to motor dysfunction in tottering.

Calcium channelopathies: voltage-gated calcium channels

Since the initial identification of native calcium currents, significant progress has been made towards our understanding of the molecular and cellular contributions of voltage-gated calcium channels in multiple physiological processes. Moreover, we are beginning to comprehend their pathophysiological roles through both naturally occurring channelopathies in humans and mice and through targeted gene deletions. The data illustrate that small perturbations in voltage-gated calcium channel function induced by genetic alterations can affect a wide variety of mammalian developmental, physiological and behavioral functions. At least in those instances wherein the channelopathies can be attributed to gain-of-function mechanisms, the data point towards new therapeutic strategies for developing highly selective calcium channel antagonists.

Elementary mechanisms producing facilitation of Cav2.1 (P/Q-type) channels

The regulation of Ca(V)2.1 (P/Q-type) channels by calmodulin (CaM) showcases the powerful Ca(2+) decoding capabilities of CaM in complex with the family of Ca(V)1-2 Ca(2+) channels. Throughout this family, CaM does not simply exert a binary on/off regulatory effect; rather, Ca(2+) binding to either the C- or N-terminal lobe of CaM alone can selectively trigger a distinct form of channel modulation. Additionally, Ca(2+) binding to the C-terminal lobe triggers regulation that appears preferentially responsive to local Ca(2+) influx through the channel to which CaM is attached (local Ca(2+) preference), whereas Ca(2+) binding to the N-terminal lobe triggers modulation that favors activation via Ca(2+) entry through channels at a distance (global Ca(2+) preference). Ca(V)2.1 channels fully exemplify these features; Ca(2+) binding to the C-terminal lobe induces Ca(2+)-dependent facilitation of opening (CDF), whereas the N-terminal lobe yields Ca(2+)-dependent inactivation of opening (CDI). In mitigation of these interesting indications, support for this local/global Ca(2+) selectivity has been based upon indirect inferences from macroscopic recordings of numerous channels. Nagging uncertainty has also remained as to whether CDF represents a relief of basal inhibition of channel open probability (P(o)) in the presence of external Ca(2+), or an actual enhancement of P(o) over a normal baseline seen with Ba(2+) as the charge carrier. To address these issues, we undertake the first extensive single-channel analysis of Ca(V)2.1 channels with Ca(2+) as charge carrier. A key outcome is that CDF persists at this level, while CDI is entirely lacking. This result directly upholds the local/global Ca(2+) preference of the lobes of CaM, because only a local (but not global) Ca(2+) signal is here present. Furthermore, direct single-channel determinations of P(o) and kinetic simulations demonstrate that CDF represents a genuine enhancement of open probability, without appreciable change of activation kinetics. This enhanced-opening mechanism suggests that the CDF evoked during action-potential trains would produce not only larger, but longer-lasting Ca(2+) responses, an outcome with potential ramifications for short-term synaptic plasticity.

Characterization of acetylcholine release and the compensatory contribution of non-Cav2.1 channels at motor nerve terminals of leaner Cav2.1-mutant mice

The severely ataxic and epileptic mouse leaner (Ln) carries a natural splice site mutation in Cacna1a, leading to a C-terminal truncation of the encoded Ca(v)2.1 alpha(1) protein. Ca(v)2.1 is a neuronal Ca(2+) channel, mediating neurotransmitter release at many central synapses and the peripheral neuromuscular junction (NMJ). With electrophysiological analyses we demonstrate severely reduced ( approximately 50%) neurotransmitter release at Ln NMJs. This equals the reduction at NMJs of Cacna1a null-mutant (Ca(v)2.1-KO) mice, which display a neurological phenotype remarkably similar to that of Ln mice. However, using selective Ca(v) channel blocking compounds we revealed a compensatory contribution profile of non-Ca(v)2.1 type channels at Ln NMJs that differs completely from that at Ca(v)2.1-KO NMJs. Our data indicate that the residual function and presence of Ln-mutated Ca(v)2.1 channels precludes presynaptic compensatory recruitment of Ca(v)1 and Ca(v)2.2 channels, and hampers that of Ca(v)2.3 channels. This is the first report directly showing at single synapses the deficits and plasticity in transmitter release resulting from the Ln mutation of Cacna1a.

Motor Deficits in Homozygous and Heterozygous P/Q-Type Calcium Channel Mutants

P/Q-type voltage-dependent Ca2+channels (VDCCs) are highly expressed in the cerebellum, and mutations of these channels are associated with disrupted motor function. Several allelic variants of the α1A pore-forming subunit of P/Q-type VDCCs have been described, and mice homozygous for these mutations exhibit gait ataxia, as do α1A knockout mice. Here we report that heterozygous α1A mutants also have a motor phenotype. Mice heterozygous for the leaner and α1A knockout mutations exhibit impaired motor learning in the vestibulo-ocular reflex (VOR), suggesting that subtle disruption of P/Q Ca2+currents is sufficient to disrupt motor function. Basal VOR and optokinetic reflex performance were normal in the heterozygotes but severely impaired in the leaner and α1A knockout homozygotes.

Migraine genetics: an update

A growing interest in genetic research in migraine has resulted in the identification of several chromosomal regions that are involved in migraine. However, the identification of mutations in the genes for familial hemiplegic migraine (FHM) forms the only true molecular genetic knowledge of migraine thus far. The increased number of mutations in the FHM1 (CACNA1A) and the FHM2 (ATP1A2) genes allow studying the relationship between genetic findings in both genes and the clinical features in patients. A wide spectrum of symptoms is seen in patients. Additional cerebellar ataxia and (childhood) epilepsy can occur in FHM1 and FHM2. Functional studies show a dysfunction in ion transport as the key factor in the pathophysiology of (familial hemiplegic) migraine that predict an increased susceptibility to cortical spreading depression--the underlying mechanism of migraine aura.

A CaV2.1 calcium channel mutationrockerreduces the number of postsynaptic AMPA receptors in parallel fiber-Purkinje cell synapses

The rocker mice are hereditary ataxic mutants that carry a point mutation in the gene encoding the CaV2.1 (P/Q-type) Ca2+ channel alpha1 subunit, and show the mildest symptoms among the reported CaV2.1 mutant mice. We studied the basic characteristics of the rocker mutant Ca2+ channel and their impacts on excitatory synaptic transmission in cerebellar Purkinje cells (PCs). In acutely dissociated PC somas, the rocker mutant channel showed a moderate reduction in Ca2+ channel current density, whereas its kinetics and voltage dependency of gating remained nearly normal. Despite the small changes in channel function, synaptic transmission in the parallel fiber (PF)-PC synapses was severely impaired. The climbing fiber inputs onto PCs showed a moderate impairment but could elicit normal complex spikes. Presynaptic function of the PF-PC synapses, however, was unexpectedly almost normal in terms of paired-pulse facilitation, sensitivity to extracellular Ca2+ concentration and glutamate concentration in synaptic clefts. Electron microscopic analyses including freeze-fracture replica labeling revealed that both the number and density of postsynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors substantially decreased without gross structural changes of the PF-PC synapses. We also observed an abnormal arborization of PC dendrites in young adult rocker mice (approximately 1 month old). These lines of evidence suggest that even a moderate dysfunction of CaV2.1 Ca2+ channel can cause substantial changes in postsynaptic molecular composition of the PF-PC synapses and dendritic structure of PCs.

Nitric oxide regulates input specificity of long-term depression and context dependence of cerebellar learning

Recent studies have shown that multiple internal models are acquired in the cerebellum and that these can be switched under a given context of behavior. It has been proposed that long-term depression (LTD) of parallel fiber (PF)–Purkinje cell (PC) synapses forms the cellular basis of cerebellar learning, and that the presynaptically synthesized messenger nitric oxide (NO) is a crucial “gatekeeper” for LTD. Because NO diffuses freely to neighboring synapses, this volume learning is not input-specific and brings into question the biological significance of LTD as the basic mechanism for efficient supervised learning. To better characterize the role of NO in cerebellar learning, we simulated the sequence of electrophysiological and biochemical events in PF–PC LTD by combining established simulation models of the electrophysiology, calcium dynamics, and signaling pathways of the PC. The results demonstrate that the local NO concentration is critical for induction of LTD and for its input specificity. Pre- and postsynaptic coincident firing is not sufficient for a PF–PC synapse to undergo LTD, and LTD is induced only when a sufficient amount of NO is provided by activation of the surrounding PFs. On the other hand, above-adequate levels of activity in nearby PFs cause accumulation of NO, which also allows LTD in neighboring synapses that were not directly stimulated, ruining input specificity. These findings lead us to propose the hypothesis that NO represents the relevance of a given context and enables context-dependent selection of internal models to be updated. We also predict sparse PF activity in vivo because, otherwise, input specificity would be lost.

The cerebellum is essential for coordinated movements. The skills for executing such movements are acquired in modules of the cerebellum, and the appropriate modules in which to store the skill for a certain movement are selected according to the environment, or the context, where the movement is made. We are interested in the molecular mechanisms that enable context-dependent cerebellar learning. In search of the key molecules, we combined established simulation models of Purkinje cells, the only output neurons in the cerebellar cortex, and constructed a new model. Using computer simulation, we found that nitric oxide is likely to have a pivotal role in context-dependent learning. Our simulation also provides insights into how sparse sensory information is coded in the cerebellar cortex. These findings have led us to propose the experimentally testable hypothesis that the relevance of a given context to learning modules is represented by the concentration of nitric oxide.

Molecular basis of inherited calcium channelopathies: role of mutations in pore-forming subunits

The pore-forming alpha subunits of voltage-gated calcium channels contain the essential biophysical machinery that underlies calcium influx in response to cell depolarization. In combination with requisite auxiliary subunits, these pore subunits form calcium channel complexes that are pivotal to the physiology and pharmacology of diverse cells ranging from sperm to neurons. Not surprisingly, mutations in the pore subunits generate diverse pathologies, termed channelopathies, that range from failures in excitation-contraction coupling to night blindness. Over the last decade, major insights into the mechanisms of pathogenesis have been derived from animals showing spontaneous or induced mutations. In parallel, there has been considerable growth in our understanding of the workings of voltage-gated ion channels from a structure-function, regulation and cell biology perspective. Here we document our current understanding of the mutations underlying channelopathies involving the voltage-gated calcium channel alpha subunits in humans and other species.

Corticotropin-Releasing Factor Immunoreactivity Increases in the Cerebellar Climbing Fibers in the Novel Ataxic Mutant Mouse, Pogo

The ataxic pogo mouse (pogo/pogo) is a novel neurological mutant, which was derived as an inbred strain (KJR/MsKist) from a Korean wild mouse. The pathological manifestations include a difficulty in maintaining a normal posture, the failure of inter-limb coordination and an inability to walk straight. In this study, we examined the distribution of corticotropin-releasing factor (CRF) immunoreactive cerebellar climbing fibres and their projections to tyrosine hydroxylase (TH) immunoreactive Purkinje cells in the cerebellum of the pogo mutant mouse using immunohistochemistry. In the pogo/pogo mouse, a subset of climbing fibres was stained more intensely for CRF than in the control. Moreover, ataxic pogo mouse, neurons of the inferior olivary nucleus projecting climbing fibres were also more intensely stained for CRF than in the control. In the pogo/pogo mouse, TH immunoreactivity was located in the Purkinje cells, whereas no TH expression was found in the control. Double immunostaining for CRF and TH in the pogo/pogo cerebellum revealed that the distribution of TH-immunoreactive Purkinje cells corresponded to terminal fields of CRF-immunoreactive climbing fibres but not to the CRF-immunoreactive mossy fibres. Therefore, we suggest that an increase of CRF level may alter the function of targeted Purkinje cells and that it is related to the ataxic phenotype in the pogo mutant mouse.

Decreases in the precision of Purkinje cell pacemaking cause cerebellar dysfunction and ataxia

Episodic ataxia type-2 (EA2) is caused by mutations in P/Q-type voltage-gated calcium channels that are expressed at high densities in cerebellar Purkinje cells. Because P/Q channels support neurotransmitter release at many synapses, it is believed that ataxia is caused by impaired synaptic transmission. Here we show that in ataxic P/Q channel mutant mice, the precision of Purkinje cell pacemaking is lost such that there is a significant degradation of the synaptic information encoded in their activity. The irregular pacemaking is caused by reduced activation of calcium-activated potassium (K(Ca)) channels and was reversed by pharmacologically increasing their activity with 1-ethyl-2-benzimidazolinone (EBIO). Moreover, chronic in vivo perfusion of EBIO into the cerebellum of ataxic mice significantly improved motor performance. Our data support the hypothesis that the precision of intrinsic pacemaking in Purkinje cells is essential for motor coordination and suggest that K(Ca) channels may constitute a potential therapeutic target in EA2.

Functional compensation by other voltage-gated Ca2+ channels in mouse basal forebrain neurons with Ca(V)2.1 mutations

Tottering (tg/tg) and leaner (tg(la)/tg(la)) mutant mice exhibit distinct mutations in the gene encoding the voltage-activated Ca(2+) channel alpha(1A) subunit (CACNA1A), the pore-forming subunit of the Ca(V)2.1 (P/Q type) Ca(2+) channels. These mice exhibit absence seizures and deficiencies in motor control and other functions. Previous work in cerebellar Purkinje neurons has shown that these mutations cause dramatic reductions in calcium channel function. Because Purkinje cell somata primarily express the Ca(V)2.1 channels, the general decrease in Ca(V)2.1 channel function is observed as a profound decrease in whole-cell current. In contrast to Purkinje cells, basal forebrain (BF) neurons express all of the Ca(2+) channel alpha(1) subunits, with Ca(V)2.1 contributing approximately 30% to the whole-cell current in wild-type (+/+) mice. Here, we show that whole-cell Ba(2+) current densities in BF neurons are not reduced in the mutant genotypes despite a reduction in the Ca(V)2.1 contribution. By blocking the different Ca(2+) channel subtypes with specific pharmacological agents, we found a significant increase in the proportion of Ca(V)1 Ca(2+) current in mutant phenotypes. There was no change in tissue mRNA expression of calcium channel subtypes Ca(V)2.1, Ca(V)2.2, Ca(V)1.2, Ca(V)1.3, and Ca(V)2.3 in the tottering and leaner mutant mice. These results suggest that Ca(V)1 channels may functionally upregulate to compensate for reduced Ca(V)2.1 function in the mutants without an increase in Ca(v)1 message. Single-cell reverse transcription polymerase chain reaction (RT-PCR) experiments in a subset of sampled neurons revealed that approximately 90% of the cells could be considered cholinergic based on choline acetyltransferase (ChAT) mRNA expression.

Developmental expression of neuronal nitric oxide synthase in P/Q-type voltage-gated calcium ion channel mutant mice, leaner and tottering

Nitric oxide (NO) is a diffusible messenger molecule produced primarily by neuronal nitric oxide synthase (nNOS) in the central nervous system. Both nNOS expression and NO production are regulated by calcium ions. Leaner and tottering mice carry a mutation in the pore forming subunit (alpha1A) of P/Q-type voltage-gated calcium ion channels, which decreases calcium ion current through the affected channels and disrupts calcium homeostasis. We have previously shown that nNOS expression is altered in adult leaner and tottering cerebella. In addition, leaner and tottering mice have been shown to have abnormal cerebellar granule cell-Purkinje cell synapses and leaner cerebellar granule cells undergo abnormal apoptosis during early postnatal development. Since NO production has been linked to several developmental roles including neuronal cell death, synaptogenesis and neuronal cell survival, our objective was to evaluate the expression of nNOS in developing leaner and tottering cerebella. Our results show that nNOS is differentially expressed in leaner and tottering cerebella compared to wild type cerebella and compared to each other. In whole cerebella, Western blotting revealed a significant increase in nNOS expression at postnatal day 12 in tottering but not leaner or wild type cerebella. At the cellular level the NADPH-diaphorase marker for nNOS revealed a significant increase in nNOS expression in basket cell interneurons in both mutant mice. nNOS expression in granule cells in the internal granule cell layer in tottering mice was increased at P12, while granule cells of leaner mice exhibited decreased nNOS expression at P20. The changes in nNOS expression at P12 did not correlate with a change in overall NO production, but rather maintained wild type NO concentrations. These findings suggest that changes in nNOS expression in the leaner and tottering cerebella are compensatory in nature with NO most likely functioning as a calcium-regulated neuroprotective/neurotrophic factor in postnatal cerebellar development.

Animal models of generalized dystonia

Dystonia is a prevalent neurological disorder characterized by abnormal co-contractions of antagonistic muscle groups that produce twisting movements and abnormal postures. The disorder may be inherited, arise sporadically, or result from brain insult. Dystonia is a heterogeneous disorder because patients may exhibit focal or generalized symptoms associated with abnormalities in many brain regions including basal ganglia and cerebellum. Elucidating the pathogenic mechanisms underlying dystonia has therefore been challenging. Animal models of dystonia exhibit similar heterogeneity and are useful for understanding pathogenesis. The neurochemical and neurophysiological abnormalities in rodents with idiopathic generalized dystonia suggest that dysfunctional output from basal ganglia, cerebellum, or from multiple systems is the cause of motor dysfunction. Findings from drug- or toxin-induced dystonia in rodents and nonhuman primates mirror the genetic models. The parallels between dystonia in humans and animals suggest that the models will continue to prove useful in determining pathogenesis. Furthermore, detailed characterization of the existing models of dystonia and the development of new models hold promise for the identification of novel therapeutics.

Differential expression of T-type calcium channels in P/Q-type calcium channel mutant mice with ataxia and absence epilepsy

Mutations in P/Q-type calcium channels generate common phenotypes in mice and humans, which are characterized by ataxia, paroxysmal dyskinesia, and absence seizures. Subsequent functional changes of T-type calcium channels in thalamus are observed in P/Q-type calcium channel mutant mice and these changes play important roles in generation of absence seizures. However, the changes in T-type calcium channel function and/or expression in the cerebellum, which may be related to movement disorders, are still unknown. The leaner mouse exhibits severe ataxia, paroxysmal dyskinesia, and absence epilepsy due to a P/Q-type calcium channel mutation. We investigated changes in T-type calcium channel expression in the leaner mouse thalamus and cerebellum using quantitative real-time polymerase chain reaction (qRT-PCR) and quantitative in situ hybridization histochemistry (ISHH). qRT-PCR analysis showed no change in T-type calcium channel alpha 1G subunit (Cav3.1) expression in the leaner thalamus, but a significant decrease in alpha 1G expression in the whole leaner mouse cerebellum. Interestingly, quantitative ISHH revealed differential changes in alpha 1G expression in the leaner cerebellum, where the granule cell layer showed decreased alpha 1G expression while Purkinje cells showed increased alpha 1G expression. To confirm these observations, the granule cell layer and the Purkinje cell layer were laser capture microdissected separately, then analyzed with qRT-PCR. Similar to the observation obtained by ISHH, the leaner granule cell layer showed decreased alpha 1G expression and the leaner Purkinje cell layer showed increased alpha 1G expression. These results suggest that differential expression of T-type calcium channels in the leaner cerebellum may be involved in the observed movement disorders.

Effects of familial hemiplegic migraine type 1 mutations on neuronal P/Q-type Ca2+ channel activity and inhibitory synaptic transmission

Inhibitory synapses play key roles in the modulatory circuitry that regulates pain signaling and generation of migraine headache. A rare, dominant form of this common disease, familial hemiplegic migraine type 1 (FHM1), arises from missense mutations in the pore-forming alpha1A subunit of P/Q-type Ca2+ channels. These channels are normally vital for presynaptic Ca2+ entry and neurotransmitter release at many central synapses, raising questions about effects of FHM1 mutations on neuronal Ca2+ influx and inhibitory and excitatory neurotransmission. We have expressed the four original FHM1 mutant channels in hippocampal neurons from alpha1A knockout mice. Whole-cell recordings indicated that FHM1 mutant channels were less effective than wild-type channels in their ability to conduct P/Q-type current, but not generally different from wild type in voltage-dependent channel gating. Ca2+ influx triggered by action potential waveforms was also diminished. In keeping with decreased channel activity, FHM1 mutant channels were correspondingly impaired in supporting the P/Q-type component of inhibitory neurotransmission. When expressed in wild-type inhibitory neurons, FHM1 mutant channels reduced the contribution of P/Q-type channels to GABAergic synaptic currents, consistent with a competition of mutant and endogenous channels for P/Q-specific slots. In all cases, N-type channels took up the burden of supporting transmission and homeostatic mechanisms maintained overall synaptic strength. The shift to reliance on N-type channels greatly increased the susceptibility to G protein-coupled modulation of neurotransmission, studied with the GABAB agonist baclofen. Thus, mutant-expressing synapses might be weakened in a heightened state of neuromodulation like that provoked by triggers of migraine such as stress.

L-type Ca2+ channels in Ca2+ channelopathies

Voltage-gated L-type Ca2+ channels (LTCCs) mediate depolarization-induced Ca2+ entry in electrically excitable cells, including muscle cells, neurons, and endocrine and sensory cells. In this review we summarize the role of LTCCs for human diseases caused by genetic Ca2+ channel defects (channelopathies). LTCC dysfunction can result from structural aberrations within pore-forming alpha1 subunits causing incomplete congenital stationary night blindness, malignant hyperthermia sensitivity or hypokalemic periodic paralysis. However, studies in mice revealed that LTCC dysfunction also contributes to neurological symptoms in Ca2+ channelopathies affecting non-LTCCs, such as Ca(v)2.1 alpha1 in tottering mice. Ca2+ channelopathies provide exciting molecular tools to elucidate the contribution of different LTCC isoforms to human diseases.

Differential expression of high voltage-activated Ca2+ channel types in the rostral reticular thalamic nucleus of the absence epileptic WAG/Rij rat

In the WAG/Rij rat, a model for human absence epilepsy, spike-wave discharges (SWD) and absence epileptic behavior develop after the age of 3 months. The rostral part of the reticular thalamic nucleus (rRTN) is involved in SWD. Ca(2+) channels play a central role in the initiation and maintenance of burst firing activity of thalamic cells. We hypothesize that a changed expression of alpha(1)-subunits of one or more high voltage-activated Ca(2+) channel types in the rRTN underlies the development of SWD. To test this hypothesis we compared 3- and 6-month-old WAG/Rij rats with nonepileptic, age-matched control rats. By immunocytochemistry, the expressions of alpha(1)1.3-, alpha(1)2.1-, alpha(1)2.2-, and alpha(1)2.3-subunits were shown in both strains, demonstrating the presence of Ca(v)1.3, Ca(v)2.1, Ca(v)2.2, and Ca(v)2.3 channels, respectively. Quantification of channel expression indicates that the development of SWD in WAG/Rij rats is concomitant with an increased expression of Ca(v)2.1 channels in the rRTN. These channels are mainly presynaptic, as revealed by double immunofluorescence involving the presynapse marker syntaxin. The mechanism by which this increase could be related to the occurrence of SWD has been discussed.

A Cacna1a knockin migraine mouse model with increased susceptibility to cortical spreading depression

Migraine is a common, disabling, multifactorial, episodic neurovascular disorder of unknown etiology. Familial hemiplegic migraine type 1 (FHM-1) is a Mendelian subtype of migraine with aura that is caused by missense mutations in the CACNA1A gene that encodes the alpha(1) subunit of neuronal Ca(v)2.1 Ca(2+) channels. We generated a knockin mouse model carrying the human pure FHM-1 R192Q mutation and found multiple gain-of-function effects. These include increased Ca(v)2.1 current density in cerebellar neurons, enhanced neurotransmission at the neuromuscular junction, and, in the intact animal, a reduced threshold and increased velocity of cortical spreading depression (CSD; the likely mechanism for the migraine aura). Our data show that the increased susceptibility for CSD and aura in migraine may be due to cortical hyperexcitability. The R192Q FHM-1 mouse is a promising animal model to study migraine mechanisms and treatments.

Eye movements of the murine P/Q calcium channel mutant rocker, and the impact of aging

Mutations in the gene encoding the ion pore of the P/Q voltage-activated calcium channel (CACNA1A) are predicted to alter synaptic transmission and dendritic excitability within cerebellar granule and Purkinje cells. Determining the relationships between these alterations, neuronal activity, and behavior may yield insight into the relationship between neuronal intrinsic properties and signal processing within the ocular motor system. Toward this end, we compared ocular motor performance in the CACNA1A mutant rocker and C57BL/6 controls. Average vertical eye position was abnormally elevated in the mutants, a finding that may be analogous to downbeat nystagmus seen in human cerebellar disorders. Fast phases of vestibular nystagmus were slowed by approximately 18% of control values. The angular vestibuloocular reflex (VOR) in darkness and light (visual VOR, or VVOR), assessed at 0.1-1.6 Hz, exhibited subnormal gains at the highest stimulus frequencies and increased phase leads at the lowest stimulus frequencies. Horizontal optokinetic responses to constant velocity drum rotation of +/-2.5-40 degrees/s exhibited minimally reduced gains. Attempts to increase VOR gain by concomitant optokinetic and vestibular stimulation were confounded by the tendency of the mice to habituate to repetitive vestibular stimulation, but attempts to induce coupling of vertical eye movements to horizontal vestibular stimulation (cross-axis adaptation) generated rapid plastic changes in controls and little effect in mutants. With the notable exceptions of the vertical elevation and optokinetic gains, the ocular motor abnormalities were stable over a broad range of animal age, a result compatible with the abnormalities arising as direct consequences of the inborn alteration in calcium channel biophysics.

Insulin-like growth factor-I improves cerebellar dysfunction but does not prevent cerebellar neurodegeneration in the calcium channel mutant mouse, leaner

The effects of insulin-like growth factor-I (IGF-I) on cerebellar dysfunction and neurodegeneration were investigated in leaner mice, which exhibit cerebellar ataxia and neurodegeneration related to P/Q-type calcium channel mutations. Leaner mice showed significantly reduced serum and cerebellar IGF-I concentrations compared to wild-type mice at postnatal day 30. Behavioral assessment of leaner mice injected with IGF-I subcutaneously for 4 weeks showed partially improved cerebellar function. Histological analysis of IGF-I treated leaner cerebella showed no difference in the number of dying Purkinje cells compared to control leaner cerebella. These results further support potential use of IGF-I as a therapeutic aid for cerebellar ataxia related to calcium channel mutations. Nonetheless, IGF-I administration does not rescue dying cerebellar neurons, which suggests that the beneficial effects of IGF-I may have been achieved through surviving cerebellar neurons.