Cerebellum-related characteristics of Scn8a-mutant mice

Advanced progress of vestibular compensation in vestibular neural networks

Vestibular compensation is the natural process of recovery that occurs with acute peripheral vestibular lesion. Here, we summarize the current understanding of the mechanisms underlying vestibular compensation, focusing on the role of the medial vestibular nucleus (MVN), the central hub of the vestibular system, and its associated neural networks. The disruption of neural activity balance between the bilateral MVNs underlies the vestibular symptoms after unilateral vestibular damage, and this balance disruption can be partially reversed by the mutual inhibitory projections between the bilateral MVNs, and their top-down regulation by other brain regions via different neurotransmitters. However, the detailed mechanism of how MVN is involved in vestibular compensation and regulated remains largely unknown. A deeper understanding of the vestibular neural network and the neurotransmitter systems involved in vestibular compensation holds promise for improving treatment outcomes and developing more effective interventions for vestibular disorders.

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.

An integrative analysis of miRNA and mRNA expression in the brains of Alzheimer's disease transgenic mice after real-world PM2.5 exposure

Fine particulate matter (PM_2.5) is associated with increased risks of Alzheimer's disease (AD), yet the toxicological mechanisms of PM_2.5 promoting AD remain unclear. In this study, wild-type and APP/PS1 transgenic mice (AD mice) were exposed to either filtered air (FA) or PM_2.5 for eight weeks with a real-world exposure system in Taiyuan, China (mean PM_2.5 concentration in the cage was 61 µg/m³). We found that PM_2.5 exposure could remarkably aggravate AD mice's ethological and brain ultrastructural damage, along with the elevation of the pro-inflammatory cytokines (IL-6 and TNF-α), Aβ-42 and AChE levels and the decline of ChAT levels in the brains. Based on high-throughput sequencing results, some differentially expressed (DE) mRNAs and DE miRNAs in the brains of AD mice after PM_2.5 exposure were screened. Using RT-qPCR, seven DE miRNAs (mmu-miR-193b-5p, 122b-5p, 466h-3p, 10b-5p, 1895, 384-5p, and 6412) and six genes (Pcdhgb8, Unc13b, Robo3, Prph, Pter, and Tbata) were evidenced the and verified. Two miRNA-target gene pairs (miR-125b-Pcdhgb8 pair and miR-466h-3p-IL-17Rα/TGF-βR2/Aβ-42/AChE pairs) were demonstrated that they were more related to PM_2.5-induced brain injury. Results of Gene Ontology (GO) pathways and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways predicted that synaptic and postsynaptic regulation, axon guidance, Wnt, MAPK, and mTOR pathways might be the possible regulatory mechanisms associated with pathological response. These revealed that PM_2.5-elevated pro-inflammatory cytokine levels and PM_2.5-altered neurotransmitter levels in AD mice could be the important causes of brain damage and proposed the promising miRNA and mRNA biomarkers and potential miRNA-mRNA interaction networks of PM_2.5-promoted AD.

Association of respiratory failure with inhibition of NaV1.6 in the phrenic nerve

As part of a drug discovery effort to identify potent inhibitors of NaV1.7 for the treatment of pain, we observed that inhibitors produced unexpected cardiovascular and respiratory effects in vivo. Specifically, inhibitors administered to rodents produced changes in cardiovascular parameters and respiratory cessation. We sought to determine the mechanism of the in vivo adverse effects by studying the selectivity of the compounds on NaV1.5, NaV1.4, and NaV1.6 in in vitro and ex vivo assays. Inhibitors lacking sufficient NaV1.7 selectivity over NaV1.6 were associated with respiratory cessation after in vivo administration to rodents. Effects on respiratory rate in rats were consistent with effects in an ex vivo hemisected rat diaphragm model and in vitro NaV1.6 potency. Furthermore, direct blockade of the phrenic nerve signaling was observed at exposures known to cause respiratory cessation in rats. Collectively, these results support a significant role for NaV1.6 in phrenic nerve signaling and respiratory function.

Social Deficits and Cerebellar Degeneration in Purkinje Cell Scn8a Knockout Mice

Mutations in the SCN8A gene encoding the voltage-gated sodium channel α-subunit Nav1. 6 have been reported in individuals with epilepsy, intellectual disability and features of autism spectrum disorder. SCN8A is widely expressed in the central nervous system, including the cerebellum. Cerebellar dysfunction has been implicated in autism spectrum disorder. We investigated conditional Scn8a knockout mice under C57BL/6J strain background that specifically lack Scn8a expression in cerebellar Purkinje cells (Scn8a ^flox/flox , L7Cre ⁺ mice). Cerebellar morphology was analyzed by immunohistochemistry and MR imaging. Mice were subjected to a battery of behavioral tests including the accelerating rotarod, open field, elevated plus maze, light-dark transition box, three chambers, male-female interaction, social olfaction, and water T-maze tests. Patch clamp recordings were used to evaluate evoked action potentials in Purkinje cells. Behavioral phenotyping demonstrated that Scn8a ^flox/flox , L7Cre ⁺ mice have impaired social interaction, motor learning and reversal learning as well as increased repetitive behavior and anxiety-like behaviors. By 5 months of age, Scn8a ^flox/flox , L7Cre ⁺ mice began to exhibit cerebellar Purkinje cell loss and reduced molecular thickness. At 9 months of age, Scn8a ^flox/flox , L7Cre ⁺ mice exhibited decreased cerebellar size and a reduced number of cerebellar Purkinje cells more profoundly, with evidence of additional neurodegeneration in the molecular layer and deep cerebellar nuclei. Purkinje cells in Scn8a ^flox/flox , L7Cre ⁺ mice exhibited reduced repetitive firing. Taken together, our experiments indicated that loss of Scn8a expression in cerebellar Purkinje cells leads to cerebellar degeneration and several ASD-related behaviors. Our study demonstrated the specific contribution of loss of Scn8a in cerebellar Purkinje cells to behavioral deficits characteristic of ASD. However, it should be noted that our observed effects reported here are specific to the C57BL/6 genome type.

SCN8A Encephalopathy: Case Report and Literature Review

Epileptic encephalopathy is a condition resulting from extreme forms of intractable childhood epilepsy. The disease can cause severe delays in cognitive, sensory, and motor function development, in addition to being fatal in some cases. Missense mutations of SCN8A, which encodes Nav1.6, one of the main voltage-gated sodium channel subunits in neurons and muscles, have been linked to early infantile SCN8A encephalopathy. Herein, we report the case of a 5-month-old girl with SCN8A encephalopathy with a novel missense mutation. Apart from intractable seizures and autistic phenotypes, the results of blood and biochemical tests, electroencephalogram (EEG) results, and brain magnetic resonance imaging (MRI) results were all normal. As the phenotypes caused by these mutations cannot be identified by any clinical, neuroimaging, or electrophysiological features, genetic sequencing should be considered to identify the underlying genetic causes. Although phenytoin is recommended as a last-resort treatment for SCN8A encephalopathy, the administration of the oxcarbazepine, instead of phenytoin, mitigated this patient's intractable seizures.

Amino acid and acetylcholine chemistry in mountain beaver cochlear nucleus and comparisons to pocket gopher, other rodents, and cat

The mountain beaver and pocket gopher are two rodents that live mostly underground in tunnel systems. Previous studies have suggested that their cochlear nucleus structure, particularly that of the dorsal cochlear nucleus (DCN), differs significantly from that of other mammals, that the hearing ability of the pocket gopher is deficient compared to that of other rodents, and that the DCN of the mountain beaver is more responsive to slow oscillations of air pressure than to sounds. We conducted some electrophysiological recordings from mountain beaver DCN and then used microchemical methods to map in mountain beaver cochlear nuclei the distributions of amino acids, including the major neurotransmitters of the brain, and enzyme activities related to the metabolism of the neurotransmitter acetylcholine, which functions in centrifugal pathways to the cochlear nucleus. Similar measurements were made for a pocket gopher cochlear nucleus. Responses to tonal stimuli were found in mountain beaver DCN. Distributions and magnitudes of neurotransmitter and related amino acids within mountain beaver and pocket gopher cochlear nuclei were not very different from those of other rodents and cat. However, the enzyme of synthesis for acetylcholine, choline acetyltransferase, had only low activities in the DCN of both mountain beaver and pocket gopher. The chemical distributions in the mountain beaver DCN support a conclusion that it corresponds to just the superficial DCN portion of other mammals. High correlations between the concentrations of γ-aminobutyrate (GABA) and glycine were found for both mountain beaver and pocket gopher cochlear nuclei, suggesting that their co-localization in cochlear nucleus synapses may be especially prominent in these animals. Previous evidence suggests convergence of somatosensory and auditory information in the DCN, and this may be especially true in animals spending most of their time underground. Our results suggest that the enlarged DCN of the mountain beaver and that of the pocket gopher are not very different from those of other rodents with respect to involvement of amino acid neurotransmitters, but they appear to have reduced cholinergic innervation.

Mutations in the Scn8a DIIS4 voltage sensor reveal new distinctions among hypomorphic and null Nav 1.6 sodium channels

Mutations in the voltage-gated sodium channel gene SCN8A cause a broad range of human diseases, including epilepsy, intellectual disability, and ataxia. Here we describe three mouse lines on the C57BL/6J background with novel, overlapping mutations in the Scn8a DIIS4 voltage sensor: an in-frame 9 bp deletion (Δ9), an in-frame 3 bp insertion (∇3) and a 35 bp deletion that results in a frameshift and the generation of a null allele (Δ35). Scn8a ^Δ9/+ and Scn8a ^∇3/+ heterozygous mutants display subtle motor deficits, reduced acoustic startle response, and are resistant to induced seizures, suggesting that these mutations reduce activity of the Scn8a channel protein, Na_v 1.6. Heterozygous Scn8a ^Δ35/+ mutants show no alterations in motor function or acoustic startle response, but are resistant to induced seizures. Homozygous mutants from each line exhibit premature lethality and severe motor impairments, ranging from uncoordinated gait with tremor (Δ9 and ∇3) to loss of hindlimb control (Δ35). Scn8a ^Δ9/Δ9 and Scn8a ^∇3/∇3 homozygous mutants also exhibit impaired nerve conduction velocity, while normal nerve conduction was observed in Scn8a ^Δ35/Δ35 homozygous mice. Our results suggest that hypomorphic mutations that reduce Na_v 1.6 activity will likely result in different clinical phenotypes compared to null alleles. These three mouse lines represent a valuable opportunity to examine the phenotypic impacts of hypomorphic and null Scn8a mutations without the confound of strain-specific differences.

A Missense Variant in SCN8A in Alpine Dachsbracke Dogs Affected by Spinocerebellar Ataxia

Spinocerebellar ataxias is an umbrella term for clinically- and neuropathologically-heterogeneous early-onset hereditary neurodegenerative diseases affecting several dog breeds. The purpose of this study is to identify the causative genetic variant associated with ataxia, tremor, and loss of balance in Alpine Dachsbracke dogs. We investigated two related litters in which four cases were reported. Neuropathology of two dogs revealed spongy degeneration associated with axonal degeneration. Combined genetic linkage and autozygosity analyses in four cases and eight related controls showed one critical disease-associated interval on chromosomes 27. Private whole-genome sequence variants of one ataxia case against 600 unrelated controls revealed one protein-changing variant within the critical interval in the SCN8A gene (c.4898G>T; p.Gly1633Val). Perfect segregation with the phenotype was confirmed by genotyping >200 Alpine Dachsbracke dogs. SCN8A encodes a voltage-gated sodium channel and the missense variant was predicted deleterious by three different in silico prediction tools. Pathogenic variants in SCN8A were previously reported in humans with ataxia, pancerebellar atrophy, and cognitive disability. Furthermore, cerebellar ataxia syndrome in the 'jolting' mutant mice is caused by a missense variant in Scn8a. Therefore, we considered the SCN8A:c.4898G>T variant to be the most likely cause for recessively inherited spinocerebellar ataxia in Alpine Dachsbracke dogs.

Normal audiogram but poor sensitivity to brief sounds in mice with compromised voltage-gated sodium channels (Scn8amedJ)

The Scn8a^medJ mutation of the gene for sodium channels at the nodes of Ranvier slows nerve conduction, resulting in motor abnormalities. This mutation is also associated with loss of spontaneous bursting activity in the dorsal cochlear nucleus. However initial tests of auditory sensitivity in mice homozygous for this mutation, using standard 400-ms tones, demonstrated normal hearing sensitivity. Further testing, reported here, revealed a severely compromised sensitivity to short-duration tones of 10 and 2 ms durations. Such a deficit might be expected to interfere with auditory functions that depend on rapid processing of auditory signals.

Cochlear ablation effects on amino acid levels in the chinchilla cochlear nucleus

Inner ear damage can lead to hearing disorders, including tinnitus, hyperacusis, and hearing loss. We measured the effects of severe inner ear damage, produced by cochlear ablation, on the levels and distributions of amino acids in the first brain center of the auditory system, the cochlear nucleus. Measurements were also made for its projection pathways and the superior olivary nuclei. Cochlear ablation produces complete degeneration of the auditory nerve, which provides a baseline for interpreting the effects of partial damage to the inner ear, such as that from ototoxic drugs or intense sound. Amino acids play a critical role in neural function, including neurotransmission, neuromodulation, cellular metabolism, and protein construction. They include major neurotransmitters of the brain - glutamate, glycine, and γ-aminobutyrate (GABA) - as well as others closely related to their metabolism and/or functions - aspartate, glutamine, and taurine. Since the effects of inner ear damage develop over time, we measured the changes in amino acid levels at various survival times after cochlear ablation. Glutamate and aspartate levels decreased by 2weeks in the ipsilateral ventral cochlear nucleus and deep layer of the dorsal cochlear nucleus, with the largest decreases in the posteroventral cochlear nucleus (PVCN): 66% for glutamate and 63% for aspartate. Aspartate levels also decreased in the lateral part of the ipsilateral trapezoid body, by as much as 50%, suggesting a transneuronal effect. GABA and glycine levels showed some bilateral decreases, especially in the PVCN. These results may represent the state of amino acid metabolism in the cochlear nucleus of humans after removal of eighth nerve tumors, which may adversely result in destruction of the auditory nerve. Measurement of chemical changes following inner ear damage may increase understanding of the pathogenesis of hearing impairments and enable improvements in their diagnosis and treatment.

Animal models of human cerebellar ataxias: a cornerstone for the therapies of the twenty-first century

Cerebellar ataxias represent a group of disabling neurological disorders. Our understanding of the pathogenesis of cerebellar ataxias is continuously expanding. A considerable number of laboratory animals with neurological mutations have been reported and numerous relevant animal models mimicking the phenotype of cerebellar ataxias are becoming available. These models greatly help dissecting the numerous mechanisms of cerebellar dysfunction, a major step for the assessment of therapeutics targeting a given deleterious pathway and for the screening of old or newly synthesized chemical compounds. Nevertheless, differences between animal models and human disorders should not be overlooked and difficulties in terms of characterization should not be occulted. The identification of the mutations of many hereditary ataxias, the development of valuable animal models, and the recent identifications of the molecular mechanisms underlying cerebellar disorders represent a combination of key factors for the development of anti-ataxic innovative therapies. It is anticipated that the twenty-first century will be the century of effective therapies in the field of cerebellar ataxias. The animal models are a cornerstone to reach this goal.