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

Abstract

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.

The Knockout of Secretin in Cerebellar Purkinje Cells Impairs Mouse Motor Coordination and Motor Learning

“…The delayed onset of both motor reflexes (Figures 4a-c) compared with Sct fl/fl littermates illustrated the effects of SCT in neurobehavioral development (Mocholi et al, 2011;Yu et al, 2008). The time window for the development of these two reflexes (approximately P6-P9) coincides with the onset of Cre activity (Zhang et al, 2004) and is an important stage for Purkinje cell development (Erickson et al, 2007). Anatomical studies provided preliminary information regarding the role of SCT in cerebellar development; eg, SCT was shown to stimulate neurite outgrowth in vitro (Kim et al, 2006) and to protect cerebellar granular cell progenitors from ethanol toxicity in mice (Hwang et al, 2009).…”

Constitutive activity of the Ghrelin receptor reduces surface expression of voltage-gated Ca2+ channels in a CaVβ-dependent manner

“…Studies that explore Ca V channel trafficking have started to contribute to knowledge in this field but there is much that we still do not know (Marangoudakis et al, 2012;Erickson et al, 2007). Activated GPCRs can promote Ca V channel removal from the plasma membrane by internalization (Simms and Zamponi, 2012) and, in some cases, GPCR and Ca V channels are internalized together (Kisilevsky and Zamponi, 2008).…”

Low-Frequency Oscillations in the Cerebellar Cortex of the Tottering Mouse

“…P/Q-type Ca 2ϩ channels are also the dominant type of voltage-gated Ca 2ϩ channels on mature PCs (Llinás et al 1992;Mintz et al 1992). In tg mice, the mutation results in an approximately 30 -40% reduction in P/Q-type current density in PCs with little alteration in channel kinetics (Erickson et al 2007;Wakamori et al 1998). In EA2 patients, the mutations also result in reduced P/Q-type Ca 2ϩ channel function (Guida et al 2001;Jen et al 2001;Jeng et al 2006;Wappl et al 2002).…”

“…The cerebellum is required for expression of the episodic motor attacks in the tg mouse and the cerebellum is highly activated during the attacks (Campbell and Hess 1998;. In addition to the reduction in P/Q-type Ca 2ϩ channel function, Ca v 1.2/1.3 (L-type) Ca 2ϩ channels are up-regulated in the tg mouse and are involved in the episodic dystonia Erickson et al 2007;Fureman et al 2002). Activating these channels with an L-type agonist evokes the episodic dystonia and blocking these channels suppresses the induction of the motor attacks Fureman et al 2002).…”