K_CaChannels as Therapeutic Targets in Episodic Ataxia Type-2

Abstract

Episodic ataxia type-2 (EA2) is an inherited movement disorder caused by mutations in the gene encoding the Ca v 2.1␣1 subunit of the P/Q-type voltage-gated calcium channel that result in an overall reduction in the P/Q-type calcium current. A consequence of these mutations is loss of precision of pacemaking in cerebellar Purkinje cells. This diminished precision reduces the information encoded by Purkinje cells and is thought to contribute to symptoms associated with this disorder. The loss of the precision of pacemaking in EA2 is the consequence of reduced activation of calcium-dependent potassium channels (K Ca ) by the smaller calcium current and in vitro can be pharmacologically restored by K Ca activators. We used a well established mouse model of EA2, the tottering (tg/tg) mouse, to examine the potential therapeutic utility of one such Food and Drug Administration (FDA)-approved compound, chlorzoxazone (CHZ). Compared with wild-type Purkinje cells, we found the firing rate of tg/tg Purkinje cells in acutely prepared cerebellar slices to be very irregular. Bath application of CHZ successfully restored the precision of pacemaking in a dose-dependent manner. Oral administration of CHZ to tg/tg mice improved their baseline motor performance and reduced the severity, frequency, and duration of episodes of dyskinesia without producing any adverse effects. We propose the use of CHZ, which is currently FDA approved as a muscle relaxant, as a safe and novel treatment of EA2.

Mutation of plasma membrane Ca 2+ ATPase isoform 3 in a family with X-linked congenital cerebellar ataxia impairs Ca 2+ homeostasis

“…Both the unspliced variant b and the C-terminally truncated a variant of PMCA3 are abundant in human cerebellum (and brain), where they are widely expressed in the main (PF-PN) cerebellar synapse and where they play a critical role in PN and granular neurons Ca 2+ homeostasis (28). As mentioned above, reduced Ca 2+ influx has been reported in the PNs of an ataxic mouse (7,8) and has been claimed to exacerbate the severity of other ataxic models (29). An ataxic phenotype (see above) has also been induced by the ablation of the PMCA2 gene in mice (12).…”

“…Genetic defects of neuronal Ca 2+ homeostasis have been associated with diverse disease phenotype. For instance, defective Ca 2+ influx in PNs has been linked to the overt ataxia of tottering mouse (7,8). To date, genetic defects of PMCAs have been demonstrated only for PMCA2 [refs.…”

Targeting potassium channels to treat cerebellar ataxia

“…A combination of chlorzoxazone and baclofen restored tonic spiking to nonfiring ATXN1[82Q] Purkinje neurons in acute cerebellar slices (Fig 2B). Chlorzoxazone is a known activator of calcium‐activated potassium (K Ca ) channels, both BK and the related small‐conductance calcium‐activated potassium (SK) channel 36, 41, 42, 43. Baclofen, a GABA B agonist, potentiates a subthreshold‐activated potassium channel current in Purkinje neurons likely mediated by G‐protein‐coupled inwardly rectifying potassium (GIRK) channels 44.…”

“…Since SKA‐31 is not easily water‐soluble, drinking water also contained 0.05% β ‐(hydroxypropyl)‐cyclodextrin and 40 μ L/L of 1N NaOH, and supplemented with up to 8% sucrose. Chlorzoxazone was dissolved in drinking water at 15 mmol/L as described previously 36. Similar to SKA‐31, drinking water containing chlorzoxazone also contained 0.05% β ‐(hydroxypropyl)‐cyclodextrin and 40 μ L/L of 1 N NaOH, and supplemented with up to 8% sucrose.…”

Pharmacological gating modulation of small- and intermediate-conductance Ca2+-activated K+channels (KCa2.x and KCa3.1)

“…Local application of 1-EBIO to the cerebellum or systemic administration of chlorzoxazone both reestablished regular pacemaking activity and partially counteracted ataxia. 54,55 While this effect is straightforward to rationalize from the mode-of-action of positive modulators, it is less clear why CyPPA, NS13001, and SKA-31 exert positive effects in rodent models of spinocerebellar ataxias (SCA2 and SCA3), which are due to gain-offunction toxicity caused by glutamine prolongation of ataxins. 41,56 One possibility is that diseased Purkinje cells -maybe independently of the molecular cause -go through phases where the highly regulated pacemaker firing is lost, for example through changes in climbing fiber input or denervation, 57,58 leading to excessive endogeneous burst firing, depolarization block, and Ca 2C overload, which may contribute to accelerated cell death.…”