Cerebellar structure and function in the murine mutant "jolting"

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.

Neuronal voltage-gated ion channels are genetic modifiers of generalized epilepsy with febrile seizures plus

Mutations in the neuronal voltage-gated sodium channel genes SCN1A and SCN2A are associated with inherited epilepsies, including genetic epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome (severe myoclonic epilepsy of infancy). The clinical presentation and severity of these epilepsies vary widely, even in people with the same mutation, suggesting the action of environmental or genetic modifiers. To gain support for the hypothesis that genetic modifiers can influence clinical presentation in patients with SCN1A-derived GEFS+, we used mouse models to study the effect of combining the human GEFS+ mutation SCN1A-R1648H with SCN2A, KCNQ2, and SCN8A mutations. Knock-in mice heterozygous for the R1648H mutation (Scn1a(RH/+)) have decreased thresholds to induced seizures and infrequent spontaneous seizures, whereas homozygotes display spontaneous seizures and premature lethality. Scn2a(Q54) transgenic mice have a mutation in Scn2a that results in spontaneous, adult-onset partial motor seizures, and mice carrying the Kcnq2-V182M mutation exhibit increased susceptibility to induced seizures, and rare spontaneous seizures as adults. Combining the Scn1a-R1648H allele with either Scn2a(Q54) or Kcnq2(V182M/+) results in early-onset, generalized tonic-clonic seizures and juvenile lethality in double heterozygous mice. In contrast, Scn8a mutants exhibit increased resistance to induced seizures. Combining the Scn1a-R1648H and Scn8a-med-jo alleles restores normal thresholds to flurothyl-induced seizures in Scn1a(RH/+) heterozygotes and improved survival of Scn1a(RH/RH) homozygotes. Our results demonstrate that variants in Scn2a, Kcnq2, and Scn8a can dramatically influence the phenotype of mice carrying the Scn1a-R1648H mutation and suggest that ion channel variants may contribute to the clinical variation seen in patients with monogenic epilepsy.

Cerebellum-related characteristics of Scn8a-mutant mice

Among ten sodium channel alpha-subunit genes mapped in human and mouse genomes, the SCN8A gene is primarily expressed in neurons and glia. Mice with two types of Scn8a null mutations--Scn8a ( med ) and Scn8a ( medTg )--live for only 21-24 days, but those with incomplete mutations-Scn8a ( medJ ) and Scn8a ( medJo )--and those with knockout of Scn8a only in cerebellar Purkinje cells live to adult age. We review here previous work on cerebellum and related regions of Scn8a mutant mice and include some newer immunohistochemical and microchemical results. The resurgent sodium current that underlies the repeated firing of Purkinje cells is reduced in Scn8a mutant and knockout mice. Purkinje cells of mutant mice have greatly reduced spontaneous activity, as do the analogous cartwheel cells of the dorsal cochlear nucleus. Up-regulation of GABA(A) receptors in regions to which Purkinje cells project may partially compensate for their decreased activity in the mutant mice. The somata of cerebellar Purkinje cells of Scn8a ( medJ ) and Scn8a ( medJo ) mice, as revealed by PEP-19 immunoreaction, are slightly smaller than normal, and their axons, especially in Scn8a ( medJo ) mice, sometimes show enlargements similar to those in other types of mutant mice. Density of GABA-like immunoreactivity is decreased in Purkinje somata and regions of termination in deep cerebellar and vestibular nuclei of Scn8a ( medJ ) mice, but measured GABA concentration is not significantly reduced in microdissected samples of these regions. The concentrations of taurine and glutamine are significantly increased in cerebellar-related regions of Scn8a ( medJ ) mice, possibly suggesting up-regulation of glial amino acid metabolism.

Impaired Motor Function in Mice With Cell-Specific Knockout of Sodium ChannelScn8a(NaV1.6) in Cerebellar Purkinje Neurons and Granule Cells

The Scn8a gene encodes the voltage-gated Na channel α subunit NaV1.6, which is widely expressed throughout the nervous system. Global null mutations that eliminate Scn8a in all cells result in severe motor dysfunction and premature death, precluding analysis of the physiological role of NaV1.6 in different neuronal types. To test the effect of cerebellar NaV1.6 on motor coordination in mice, we used the Cre-lox system to eliminate Scn8a expression exclusively in Purkinje neurons (Purkinje KO) and/or granule neurons (granule KO). Whereas granule KO mice had only minor behavioral defects, adult Purkinje KO mice exhibited ataxia, tremor, and impaired coordination. These disorders were exacerbated in double mutants lacking Scn8a in both Purkinje and granule cells (double KO). In Purkinje cells isolated from adult Purkinje KO and double KO but not granule KO mice, the ratio of resurgent-to-transient tetrodotoxin- (TTX)-sensitive Na current amplitudes decreased from ∼15 to ∼5%. In cerebellar slices, Purkinje cell spontaneous and maximal firing rates were reduced 10-fold and twofold relative to control in Purkinje KO and double KO but not granule KO mice. Additionally, short-term plasticity of high-frequency parallel fiber EPSCs was altered relative to control in Purkinje KO and double KO but not granule KO mice. These data suggest that the specialized kinetics of Purkinje Na channels depend directly on Scn8a expression. The loss of these channels leads to a decrease in Purkinje cell firing rates as well as a modification of the synaptic properties of afferent parallel fibers, with the ultimate consequence of disrupting motor behavior.

Sodium channel Na(v)1.6 is expressed along nonmyelinated axons and it contributes to conduction

Nodes of Ranvier in myelinated fibers exhibit a complex architecture in which specific molecules organize in distinct nodal, paranodal and juxtaparanodal domains to support saltatory conduction. The clustering of sodium channel Na(v)1.6 within the nodal membrane has led to its identification as the major nodal sodium channel in myelinated axons. In contrast, much less is known about the molecular architecture of nonmyelinated fibers. In the present study, Na(v)1.6 is shown to be a significant component of nonmyelinated PNS axons. In DRG C-fibers, Na(v)1.6 is distributed continuously from terminal receptor fields in the skin to the dorsal root entry zone in the spinal cord. Na(v)1.6 is also present in the nerve endings of corneal C-fibers. Analysis of compound action potential recordings from wildtype and med mice, which lack Na(v)1.6, indicates that Na(v)1.6 plays a functional role in nonmyelinated fibers where it contributes to action potential conduction. These observations indicate that Na(v)1.6 functions not only in saltatory conduction in myelinated axons but also in continuous conduction in nonmyelinated axons.

Ankyrin-G coordinates assembly of the spectrin-based membrane skeleton, voltage-gated sodium channels, and L1 CAMs at Purkinje neuron initial segments

The axon initial segment is an excitable membrane highly enriched in voltage-gated sodium channels that integrates neuronal inputs and initiates action potentials. This study identifies Nav1.6 as the voltage-gated sodium channel isoform at mature Purkinje neuron initial segments and reports an essential role for ankyrin-G in coordinating the physiological assembly of Nav1.6, betaIV spectrin, and the L1 cell adhesion molecules (L1 CAMs) neurofascin and NrCAM at initial segments of cerebellar Purkinje neurons. Ankyrin-G and betaIV spectrin appear at axon initial segments by postnatal day 2, whereas L1 CAMs and Nav1.6 are not fully assembled at continuous high density along axon initial segments until postnatal day 9. L1 CAMs and Nav1.6 therefore do not initiate protein assembly at initial segments. betaIV spectrin, Nav1.6, and L1 CAMs are not clustered in adult Purkinje neuron initial segments of mice lacking cerebellar ankyrin-G. These results support the conclusion that ankyrin-G coordinates the physiological assembly of a protein complex containing transmembrane adhesion molecules, voltage-gated sodium channels, and the spectrin membrane skeleton at axon initial segments.

Exon organization, coding sequence, physical mapping, and polymorphic intragenic markers for the human neuronal sodium channel gene SCN8A

The voltage-gated sodium channel SCN8A is associated with inherited neurological disorders in the mouse that include ataxia, dystonia, severe muscle weakness, and paralysis. We report the complete coding sequence and exon organization of the human SCN8A gene. The predicted 1980 amino acid residues are distributed among 28 exons, including two pairs of alternatively spliced exons. The SCN8A protein is evolutionarily conserved, with 98.5% amino acid sequence identity between human and mouse. Consensus sites for phosphorylation of serine/threonine and tyrosine residues are present in cyoplasmic loop domains. The polymorphic (CA)n microsatellite marker D12S2211, with PIC = 0.68, was isolated from intron 10C of SCN8A. Single nucleotide polymorphisms in intron 19 and exon 22 were also identified. We localized SCN8A to chromosome band 12q13.1 by physical mapping on a YAC contig. The cDNA clone CSC-1 was reported by others to be a cardiac-specific sodium channel, but sequence comparison demonstrates that it is derived from exon 24 of human SCN8A. The genetic information described here will be useful in evaluating SCN8A as a candidate gene for human neurological disease.

AnkyrinG is required for clustering of voltage-gated Na channels at axon initial segments and for normal action potential firing

Voltage-gated sodium channels (NaCh) are colocalized with isoforms of the membrane-skeletal protein ankyrinG at axon initial segments, nodes of Ranvier, and postsynaptic folds of the mammalian neuromuscular junction. The role of ankyrinG in directing NaCh localization to axon initial segments was evaluated by region-specific knockout of ankyrinG in the mouse cerebellum. Mutant mice exhibited a progressive ataxia beginning around postnatal day P16 and subsequent loss of Purkinje neurons. In mutant mouse cerebella, NaCh were absent from axon initial segments of granule cell neurons, and Purkinje cells showed deficiencies in their ability to initiate action potentials and support rapid, repetitive firing. Neurofascin, a member of the L1CAM family of ankyrin-binding cell adhesion molecules, also exhibited impaired localization to initial segments of Purkinje cell neurons. These results demonstrate that ankyrinG is essential for clustering NaCh and neurofascin at axon initial segments and is required for physiological levels of sodium channel activity.

The sodium channel Scn8a is the major contributor to the postnatal developmental increase of sodium current density in spinal motoneurons

Sodium currents were recorded from motoneurons that were isolated from mice at postnatal days 0-8 (P0-P8) and maintained in culture for 12-24 hr. Motoneurons from normal mice exhibited a more than threefold increase in peak sodium current density from P0 to P8. For mice lacking a functional Scn8a sodium channel gene, motoneuronal sodium current density was comparable at P0 to that of normal mice but failed to increase from P0 to P8. The absence of Scn8a sodium channels is associated with the phenotype "motor end plate disease," which is characterized by a progressive neuromuscular failure and is fatal by 3-4 postnatal weeks. Thus, it appears that the development and function of mature motoneurons depends on the postnatal induction of Scn8a expression.

Biochemical, pathological, and clinical response to transplantation of normal bone marrow cells into acid sphingomyelinase-deficient mice

BACKGROUND

Acid sphingomyelinase knock-out (ASMKO) mice are a model of types A and B Niemann-Pick disease. In the present study, we evaluated whether bone marrow transplantation (BMT) carried out on newborn ASMKO mice could prevent or alter the Niemann-Pick disease phenotype.

METHODS

Previous work from our laboratory had shown that ASMKO mice were highly susceptible to irradiation-induced death. Therefore, we preconditioned 1-day-old ASMKO (n=35) mice with a "sublethal" dose of 200 cGy of total body irradiation before BMT. The transplantation effects were then analyzed by biochemical, pathological, and clinical approaches.

RESULTS

Engraftment ranging from 7% to 100% was achieved in 97% of the transplanted animals. Growth of the engrafted animals was improved, and their survival was increased (from a mean of 5 months to 9 months). The onset of ataxia also was delayed in most of the engrafted animals. In accordance with these observations, biochemical and pathological analysis revealed significant changes in the transplanted group as compared with nontransplanted animals. Lipid storage was reduced in several organs, and there was evidence of histologic improvement seen throughout the reticuloendothelial system, even in animals that were engrafted as low as 14%. In the central nervous system, lipid storage also was reduced, and the Purkinje cells, which are almost absent in ASMKO mice, were present in certain areas of the transplanted animals cerebella.

CONCLUSIONS

These results demonstrated that BMT could alter the pathologic phenotype in ASMKO mice, but that this procedure alone was not sufficient to elicit a complete therapeutic effect.

Altered subthreshold sodium currents and disrupted firing patterns in Purkinje neurons of Scn8a mutant mice

Sodium currents and action potentials were characterized in Purkinje neurons from ataxic mice lacking expression of the sodium channel Scn8a. Peak transient sodium current was approximately 60% of that in normal mice, but subthreshold sodium current was affected much more. Steady-state current elicited by voltage ramps was reduced to approximately 30%, and resurgent sodium current, an unusual transient current elicited on repolarization following strong depolarizations, was reduced to 8%-18%. In jolting mice, with a missense mutation in Scn8a, steady-state and resurgent current were also reduced, with altered voltage dependence and kinetics. Both spontaneous firing and evoked bursts of spikes were diminished in cells from null and jolting mice. Evidently Scn8a channels carry most subthreshold sodium current and are crucial for repetitive firing.

Molecular characterization of the sodium channel subunits expressed in mammalian cerebellar Purkinje cells

Inactivating and noninactivating Na+ conductances are known to generate, respectively, the rising phase and the prolonged plateau phase of cerebellar Purkinje cell (PC) action potentials. These conductances have different voltage activation levels, suggesting the possibility that two distinct types of ion channels are involved. Single Purkinje cell reverse transcription-PCR from guinea pig cerebellar slices identified two Na+ channel alpha subunit transcripts, the orthologs of RBI (rat brain I) and Nach6/Scn8a. The latter we shall name CerIII. In situ hybridization histochemistry in rat brain demonstrated broad CerIII expression at high levels in many neuronal groups in the brain and spinal cord, with little if any expression in white matter, or nerve tracts. RBII (rat brain II), the most commonly studied recombinant Na+ channel alpha subunit is not expressed in PCs. As the absence of Scn8a has been correlated with motor endplate disease (med), in which transient sodium currents are spared, RBI appears to be responsible for the transient sodium current in PC. Conversely, jolting mice with a mutated Scn8a message demonstrates PC abnormalities in rapid, simple spike generation, linking CerIII to the persistent sodium current.

Insertional mutation of the motor endplate disease (med) locus on mouse chromosome 15

Homozygous transgenic mice from line A4 have an early-onset progressive neuromuscular disorder characterized by paralysis of the rear limbs, muscle atrophy, and lethality by 4 weeks of age. The transgene insertion site was mapped to distal chromosome 15 close to the locus motor endplate disease (med). The sequence of mouse DNA flanking the insertion site junctions was determined. A small (< 20 kb) deletion was detected at the insertion site, with no evidence of additional rearrangement of the chromosomal DNA. Noncomplementation of the transgene-induced mutation and med was demonstrated in a cross with medJ/+mice. The new allele is designated medTgNA4Bs (medtg). The homologous human locus MED was assigned to chromosome 12. Synaptotagmin 1 and contactin 1 were eliminated as candidate genes for the med mutation. The transgene-induced allele provides molecular access to the med gene, whose function is required for synaptic transmission at the neuromuscular junction and long-term survival of cerebellar Purkinje cells.

A novel non-ataxic guinea pig strain with cerebrocortical and cerebellar abnormalities

This is the first description of GS guinea pigs, a partially inbred, non-ataxic, albino Peruvian (long-hair) strain with abnormal motor behavior and seizures. GS guinea pigs show gross and microscopic cerebellar and microscopic cerebrocortical abnormalities compared to Hartley strain animals. There is little difference between GS and short-hair guinea pig strains in Purkinje cell function, electrically evoked Ca2+ transients or immune responsiveness. The GS strain may prove useful in studying altered functions of the cerebellum in human disease.

Physiological and biochemical studies on the cerebellar cortex of the murine mutants "jolting" and "motor end-plate disease"

The activity of cerebellar Purkinje cells in the murine mutants "jolting" and "med" has been determined using extracellular electrical recordings in vivo and in isolated cerebellar slices. Most of the cells in the mutant brains failed to generate simple spontaneous action potentials, but they responded to climbing fibre inputs by generating complex potentials. The few mutant cells that were spontaneously active exhibited much lower firing frequencies than normal cells, and interval histograms of spontaneous activity were skewed towards longer intervals. The silent cells in mutant cerebellar slices could be activated by direct intracellular stimulation, by antidromic excitation and by the application of glutamate and high [K+]0. Activity was not restored by the application of bicuculline. It seems that the failure of the cerebellar Purkinje cells to generate simple spontaneous action potentials is not due to an inherent inexcitability of the soma or to the excessive activity of GABA-ergic inputs onto the cells. It is suggested that an abnormality in the behaviour of a Ca2+ channel is the most likely origin for the deficit in these mutant mice.