A channelopathy contributes to cerebellar dysfunction in a model of multiple sclerosis.

OBJECTIVE: Cerebellar dysfunction in multiple sclerosis (MS) contributes significantly to disability, is relatively refractory to symptomatic therapy, and often progresses despite treatment with disease-modifying agents. We previously observed that sodium channel Nav1.8, whose expression is normally restricted to the peripheral nervous system, is present in cerebellar Purkinje neurons in a mouse model of MS (experimental autoimmune encephalomyelitis [EAE]) and in humans with MS. Here, we tested the hypothesis that upregulation of Nav1.8 in cerebellum in MS and EAE has functional consequences contributing to symptom burden. METHODS: Electrophysiology and behavioral assessment were performed in a new transgenic mouse model overexpressing Nav1.8 in Purkinje neurons. We also measured EAE symptom progression in mice lacking Nav1.8 compared to wild-type littermates. Finally, we administered the Nav1.8-selective blocker A803467 in the context of previously established EAE to determine reversibility of MS-like deficits. RESULTS: We report that, in the context of an otherwise healthy nervous system, ectopic expression of Nav1.8 in Purkinje neurons alters their electrophysiological properties, and disrupts coordinated motor behaviors. Additionally, we show that Nav1.8 expression contributes to symptom development in EAE. Finally, we demonstrate that abnormal patterns of Purkinje neuron firing and MS-like deficits in EAE can be partially reversed by pharmacotherapy using a Nav1.8-selective blocker. INTERPRETATION: Our results add to the evidence that a channelopathy contributes to cerebellar dysfunction in MS. Our data suggest that Nav1.8-specific blockers, when available for humans, merit study in MS.

Alternative splicing may contribute to time-dependent manifestation of inherited erythromelalgia.

The Na(v)1.7 sodium channel is preferentially expressed in nocioceptive dorsal root ganglion and sympathetic ganglion neurons. Gain-of-function mutations in Na(v)1.7 produce the nocioceptor hyperexcitability underlying inherited erythromelalgia, characterized in most kindreds by early-age onset of severe pain. Here we describe a mutation (Na(v)1.7-G616R) in a pedigree with adult-onset of pain in some family members. The mutation shifts the voltage-dependence of channel fast-inactivation in a depolarizing direction in the adult-long, but not in the neonatal-short splicing isoform of Na(v)1.7 in dorsal root ganglion neurons. Altered inactivation does not depend on the age of the dorsal root ganglion neurons in which the mutant is expressed. Expression of the mutant adult-long, but not the mutant neonatal-short, isoform of Na(v)1.7 renders dorsal root ganglion neurons hyperexcitable, reducing the current threshold for generation of action potentials, increasing spontaneous activity and increasing the frequency of firing in response to graded suprathreshold stimuli. This study shows that a change in relative expression of splice isoforms can contribute to time-dependent manifestation of the functional phenotype of a sodium channelopathy.

Erythromelalgia mutation L823R shifts activation and inactivation of threshold sodium channel Nav1.7 to hyperpolarized potentials.

Erythromelalgia (also termed erythermalgia) is a neuropathic pain syndrome, characterized by severe burning pain combined with redness in the extremities, triggered by mild warmth. The inherited form of erythromelalgia (IEM) has recently been linked to mutations in voltage-gated sodium channel Nav1.7, which is expressed in peripheral nociceptors. Here, we used whole-cell voltage-clamp recordings in HEK293 cells to characterize the IEM mutation L823R, which introduces an additional positive charge into the S4 voltage sensor of domain II. The L823R mutation produces an approximately 15mV hyperpolarizing shift in the midpoint of activation and also affects the activation slope factor. Closing of the channel from the open state (deactivation) is slowed, increasing the likelihood of the channel remaining in the open state. The L823R mutation induces a approximately 10mV hyperpolarizing shift in fast-inactivation. L823R is the only naturally-occurring IEM mutation studied thus far to shift fast-inactivation to more negative potentials. We conclude that introduction of an additional charge into the S4 segment of domain II of Nav1.7 leads to a pronounced hyperpolarizing shift of activation, a change that is expected to increase nociceptor excitability despite the hyperpolarizing shift in fast-inactivation, which is unique among the IEM mutations.

Early- and late-onset inherited erythromelalgia: genotype-phenotype correlation.

Inherited erythromelalgia (IEM), an autosomal dominant disorder characterized by severe burning pain in response to mild warmth, has been shown to be caused by gain-of-function mutations of sodium channel Na(v)1.7 which is preferentially expressed within dorsal root ganglion (DRG) and sympathetic ganglion neurons. Almost all physiologically characterized cases of IEM have been associated with onset in early childhood. Here, we report the voltage-clamp and current-clamp analysis of a new Na(v)1.7 mutation, Q10R, in a patient with clinical onset of erythromelalgia in the second decade. We show that the mutation in this patient hyperpolarizes activation by only -5.3 mV, a smaller shift than seen with early-onset erythromelalgia mutations, but similar to that of I136V, another mutation that is linked to delayed-onset IEM. Using current-clamp, we show that the expression of Q10R induces hyperexcitability in DRG neurons, but produces an increase in excitability that is smaller than the change produced by I848T, an early-onset erythromelalgia mutation. Our analysis suggests a genotype-phenotype relationship at three levels (clinical, cellular and molecular/ion channel), with mutations that produce smaller effects on sodium channel activation being associated with a smaller degree of DRG neuron excitability and later onset of clinical signs.

Home safety in inner cities: prevalence and feasibility of home safety-product use in inner-city housing.

OBJECTIVES: Residential injuries cause significant morbidity and mortality in infants and young children. The American Academy of Pediatrics recommends initiating injury-prevention counseling during health supervision visits in the first 6 months of life. The objectives of this study were to describe and compare self-reported and observed home safety practices in urban, low-income families who were expecting or had a child <12 months old and to assess the feasibility of using safety products depending on the design and repair of urban homes. PARTICIPANTS AND METHODS: Women who were pregnant or had an infant <12 months old and who were enrolled in East Baltimore's Healthy Start home-visiting program were eligible for the study. For this pilot project, we used a prospective predesign/postdesign. Maternal self-report and investigator home observations documented the use of working smoke alarms on each level of the home, stair gates or doors blocking the top and bottom of all staircases, adult medication storage in locked cabinets, and the environmental feasibility of safety-product use. RESULTS: Home safety practices were higher by maternal self-report than by investigator observation. Fifty-five percent of families who reported a working smoke alarm on every level of the home had nonworking or absent smoke alarms noted during investigator observation. Of assessed staircases, 67% could not accommodate a wall-mounted gate at the top of the stairs, and 38% could not accommodate a pressure-mounted gate at the bottom of the stairs. Although most families reported locked storage of medications, 77% had unlocked medication storage documented during home observation. CONCLUSIONS: In this sample of urban families, implementation of American Academy of Pediatrics-recommended safety practices is low. The structural design of urban homes may be a significant barrier to home safety-product use. The American Academy of Pediatrics Injury Prevention Program sheets, manufacturers of safety products, and legislators need to address injury-prevention issues unique to urban, low-income families.