Ablation of P/Q-type Ca ²⁺ channel currents, altered synaptic transmission, and progressive ataxia in mice lacking the α _1A -subunit

Abstract

The Ca 2؉ channel ␣1A-subunit is a voltage-gated, pore-forming membrane protein positioned at the intersection of two important lines of research: one exploring the diversity of Ca 2؉ channels and their physiological roles, and the other pursuing mechanisms of ataxia, dystonia, epilepsy, and migraine. ␣1A-Subunits are thought to support both P-and Q-type Ca 2؉ channel currents, but the most direct test, a null mutant, has not been described, nor is it known which changes in neurotransmission might arise from elimination of the predominant Ca 2؉ delivery system at excitatory nerve terminals. We generated ␣1A-deficient mice (␣1A ؊͞؊ ) and found that they developed a rapidly progressive neurological deficit with specific characteristics of ataxia and dystonia before dying Ϸ3-4 weeks after birth. P-type currents in Purkinje neurons and P-and Q-type currents in cerebellar granule cells were eliminated completely whereas other Ca 2؉ channel types, including those involved in triggering transmitter release, also underwent concomitant changes in density. Synaptic transmission in ␣1A ؊͞؊ hippocampal slices persisted despite the lack of P͞Q-type channels but showed enhanced reliance on N-type and R-type Ca 2؉ entry. The ␣1A ؊͞؊ mice provide a starting point for unraveling neuropathological mechanisms of human diseases generated by mutations in ␣1A. The ␣ 1A -subunit, the most abundant ␣ 1 -subunit in vertebrate brain (1), mediates Ca 2ϩ influx across presynaptic and somatodendritic membranes, thereby triggering fast neurotransmitter release and other key neuronal responses (2-5). Because of its high expression levels in the brain, the ␣ 1A -subunit was the first representative of its subclass to be isolated by cDNA cloning (1, 6). This predominantly neuronal subclass also includes ␣ 1B (N-type Ca 2ϩ channel) and ␣ 1E [possibly R-type Ca 2ϩ channel (7-9)] and is referred to as ABE or Ca V 2. There is no information to date on the behavioral or electrophysiological consequences of deleting a member of the ABE subfamily.␣ 1A Transcripts are widely distributed in rat (10) and human brain (11), most prominently in cell body layers in cerebellum and hippocampus. At the subcellular level, ␣ 1A immunoreactivity has been found in cell bodies, dendrites, and presynaptic terminals (12). Less clear has been the role of ␣ 1A in supporting Ca 2ϩ channel components defined by biophysical and pharmacological criteria. In either Xenopus oocytes (13, 14) or HEK293 cells (15), expression of ␣ 1A -subunits along with ancillary ␣ 2 ͞␦-and ␤-subunits generated currents with properties closely resembling the Q-type current found in cerebellar granule cells (8) and much less the P-type current first described in cerebellar Purkinje neurons by Llinás and colleagues (16,17). Unlike native P-type channels (18), the expressed currents showed pronounced inactivation during sustained depolarizations and responded to -agatoxin IVA (-Aga-IVA) at half-blocking doses of Ϸ100 nM, not Ϸ1 nM (13). Various explanations for the discrepancies have been advanced...

γ-Band deficiency and abnormal thalamocortical activity in P/Q-type channel mutant mice

“…The heterozygous mice had the same phenotype as previously described (25,26) and were combined with the WT into one control group, which here are called WT. We used thalamocortical slices in combination with VSDI to study the patterns of both putamen/striatum and cortical activities while stimulating ventrobasal (VB) thalamic nucleus at both 40-and 10-Hz frequencies.…”

“…Although the reason for their death is multifactorial, including inanition, synaptic transmission deficits clearly play a central role. Although other types of VGCCs support synaptic transmission in these mutant animals (25)(26)(27)(28), clear deficiencies were observed in the dynamics of synaptic transmission that are related to changes in the presynaptic calcium current (26,29,30). There also is an increment in the density of T-type VGCCs that support low-threshold (LVA) action potentials in the absence of P/Q channels (30).…”

“…A murine model lacking Ca V 2.1 VGCC has been described as suffering from several neurological problems (25). Indeed, near postnatal day (P) 10, the animals begin to have difficulty walking, have absence seizures, and are ataxic and dystonic.…”

Static stabilometry in patients with migraine and tension‐type headache during a headache‐free period

“…It remains to be elucidated how the genetic mutation in the Ca channel causes migraine headaches, however, ablation of the P/Q type Ca channel current has been reported to alter synaptic transmission. 31 Using static stabilometric apparatus, we demonstrated a significant increase in body sway during the EC condition in migraine patients but not in TH patients. Regarding the existence of the subtle but significant dysfunction in vestibulospinal systems in migraine patients, the hypothesis that migraine is a channelopathy and may be associated with Ca the channel gene appears to be a promising strategy.…”

Familial Hemiplegic Migraine

“…Deletion of the Ca V 2.1␣ 1 mouse gene leads to severe cerebellar ataxia and dystonia together with selective progressive cerebellar degeneration. 22,23 Different mouse strains with spontaneous Ca V 2.1␣ 1 mutations all suffer from ataxia and exhibit reduced P/Q-type current in Purkinje cells. 24 Moreover, null Ca V 2.1 Ϫ/Ϫ mice and the majority of the spontaneous mutants harboring loss-of-function mutations in Ca V 2.1 show absence seizures.…”