Effects of age-related loss of P/Q-type calcium channels in a mice model of peripheral nerve injury.

We analyzed the role of P/Q-type calcium channels in sciatic nerve regeneration after lesion induced by chronic constriction injury (CCI) in heterozygous null mutant mice lacking the CaV2.1α1 subunit of these channels (Cacna1a+/-). Compared with wild type, Cacna1a+/- mice showed an initial reduction of the CCI-induced allodynia, indicating a reduced pain perception, but they also evidenced a lack of recovery over time, with atrophy of the injured hindpaw still present 3 months after CCI when wild-type mice fully recovered. In parallel, Cacna1a+/- mice exhibited an early onset of age-dependent loss of P/Q-type channels, which can be responsible for the lack of functional recovery. Moreover, Cacna1a+/- mice showed an early age-dependent reduction of muscular strength, as well as of Schwann cells proliferation and sciatic nerve remyelination. This study demonstrates the important role played by P/Q-type channels in recovery from nerve injury and has important implications for the knowledge of age-related processes.

Stable respiratory activity requires both P/Q-type and N-type voltage-gated calcium channels.

P/Q-type voltage-gated calcium channels (Ca(v)2.1) play critical presynaptic and postsynaptic roles throughout the nervous system and have been implicated in a variety of neurological disorders. Here we report that mice with a genetic ablation of the Ca(v)2.1 pore-forming α(1A) subunit (α(1A)⁻/⁻) encoded by CACNA1a (Jun et al., 1999) suffer during postnatal development from increasing breathing disturbances that lead ultimately to death. Breathing abnormalities include decreased minute ventilation and a specific loss of sighs, which was associated with lung atelectasis. Similar respiratory alterations were preserved in the isolated in vitro brainstem slice preparation containing the pre-Bötzinger complex. The loss of Ca(v)2.1 was associated with an alteration in the functional dependency on N-type calcium channels (Ca(v)2.2). Blocking N-type calcium channels with conotoxin GVIA had only minor effects on respiratory activity in slices from control (CT) littermates, but abolished respiratory activity in all slices from α(1A)⁻/⁻ mice. The amplitude of evoked EPSPs was smaller in inspiratory neurons from α(1A)⁻/⁻ mice compared with CTs. Conotoxin GVIA abolished all EPSPs in inspiratory neurons from α(1A)⁻/⁻ mice, while the EPSP amplitude was reduced by only 30% in CT mice. Moreover, neuromodulation was significantly altered as muscarine abolished respiratory network activity in α(1A)⁻/⁻ mice but not in CT mice. We conclude that excitatory synaptic transmission dependent on N-type and P/Q-type calcium channels is required for stable breathing and sighing. In the absence of P/Q-type calcium channels, breathing, sighing, and neuromodulation are severely compromised, leading to early mortality.

Subthreshold membrane potential oscillations in inferior olive neurons are dynamically regulated by P/Q- and T-type calcium channels: a study in mutant mice.

The role of P/Q- and T-type calcium channels in the rhythmic oscillatory behaviour of inferior olive (IO) neurons was investigated in mutant mice. Mice lacking either the CaV2.1 gene of the pore-forming alpha1A subunit for P/Q-type calcium channel, or the CaV3.1 gene of the pore-forming alpha1G subunit for T-type calcium channel were used. In vitro intracellular recording from IO neurons reveals that the amplitude and frequency of sinusoidal subthreshold oscillations (SSTOs) were reduced in the CaV2.1-/- mice. In the CaV3.1-/- mice, IO neurons also showed altered patterns of SSTOs and the probability of SSTO generation was significantly lower (15%, 5 of 34 neurons) than that of wild-type (78%, 31 of 40 neurons) or CaV2.1-/- mice (73%, 22 of 30 neurons). In addition, the low-threshold calcium spike and the sustained endogenous oscillation following rebound potentials were absent in IO neurons from CaV3.1-/- mice. Moreover, the phase-reset dynamics of oscillatory properties of single neurons and neuronal clusters in IO were remarkably altered in both CaV2.1-/- and CaV3.1-/- mice. These results suggest that both alpha1A P/Q- and alpha1G T-type calcium channels are required for the dynamic control of neuronal oscillations in the IO. These findings were supported by results from a mathematical IO neuronal model that incorporated T and P/Q channel kinetics.

Altered synaptic synchrony in motor nerve terminals lacking P/Q-calcium channels.

The variance in synaptic delays among endplate potentials events (referred here as jitter) was measured to study the contribution of voltage dependent calcium channels to transmission synchronicity in neuromuscular synapses from wild type and alpha-1A knockout mice (i.e., lacking P/Q type calcium channels). Knockout synapses presented higher jitter values than wild type ones under a wide range of extracellular calcium concentration ([Ca2+]o) values. Accordingly, wild type synapses showed less synchronic neurotransmitter release when P/Q type calcium channels were partially blocked as well as under lower [Ca2+]o. In the knockout synapses, N-type calcium channels mediated neurotransmitter release in a more temporally precise way than the R-type ones. Our results suggest that the type of calcium channels mediating transmitter release influenced the degree of synaptic synchrony. Thus, these results provide insight on the mechanisms underlying several pathologies associated with P/Q type calcium channels.

Eye movements of the murine P/Q calcium channel mutant tottering, and the impact of aging.

Mice carrying mutations of the gene encoding the ion pore of the P/Q calcium channel (Cacna1a) are an instance in which cerebellar dysfunction may be attributable to altered electrophysiology and thus provide an opportunity to study how neuronal intrinsic properties dictate signal processing in the ocular motor system. P/Q channel mutations can engender multiple effects at the single neuron, circuit, and behavioral levels; correlating physiological and behavioral abnormalities in multiple allelic strains will ultimately facilitate determining which alterations of physiology are responsible for specific behavioral aberrations. We used videooculography to quantify ocular motor behavior in tottering mutants aged 3 mo to 2 yr and compared their performance to data previously obtained in the allelic mutant rocker and C57BL/6 controls. Tottering mutants shared numerous abnormalities with rocker, including upward deviation of the eyes at rest, increased vestibuloocular reflex (VOR) phase lead at low stimulus frequencies, reduced VOR gain at high stimulus frequencies, reduced gain of the horizontal and vertical optokinetic reflex, reduced time constants of the neural integrator, and reduced plasticity of the VOR as assessed in a cross-axis training paradigm. Unlike rocker, young tottering mutants exhibited normal peak velocities of nystagmus fast phases, arguing against a role for neuromuscular transmission defects in the attenuation of compensatory eye movements. Tottering also differed by exhibiting directional asymmetries of the gains of optokinetic reflexes. The data suggest at least four pathophysiological mechanisms (two congenital and two acquired) are required to explain the ocular motor deficits in the two Cacna1a mutant strains.

Altered properties of quantal neurotransmitter release at endplates of mice lacking P/Q-type Ca2+ channels.

Transmission at the mouse neuromuscular junction normally relies on P/Q-type channels, but became jointly dependent on both N- and R-type Ca(2+) channels when the PQ-type channel alpha(1A) subunit was deleted. R-type channels lay close to Ca(2+) sensors for exocytosis and I(K(Ca)) channel activation, like the P/Q-type channels they replaced. In contrast, N-type channels were less well localized, but abundant enough to influence secretion strongly, particularly when action potentials were prolonged. Our data suggested that active zone structures may select among multiple Ca(2+) channels in the hierarchy P/Q >R >N. The alpha(1A)-/- neuromuscular junction displayed several other differences from wild-type: lowered quantal content but greater ability to withstand reductions in the Ca(2+)/Mg(2+) ratio, and little or no paired-pulse facilitation, the latter findings possibly reflecting compensatory mechanisms at individual release sites. Changes in presynaptic function were also associated with a significant reduction in the size of postsynaptic acetylcholine receptor clusters.

Episodic ataxia type 2. Three novel truncating mutations and one novel missense mutation in the CACNA1A gene.

We analysed the CACNA1A gene, located on chromosome 19p13, in three unrelated families and one sporadic case with episodic ataxia type 2 (EA-2). In two of the families and the sporadic patient, novel truncating mutations, which disrupt the reading frame and result in a premature stop of the CACNA1A protein, were identified in exons 14, 16 and 26. In the remaining family, a novel missense mutation (H253Y) was found. Of the twenty two EA-2 mutations identified thus far, including those of the present study, seventeen are truncating mutations and five are missense mutations, all resulting in an EA-2 clinical phenotype.

Decreased gene expression of calretinin and ryanodine receptor type 1 in tottering mice.

Tottering mice are a spontaneously occurring animal model of human absence epilepsy. They carry a mutation in the P/Q-type calcium channel alpha1A subunit gene which is highly expressed by cerebellar Purkinje cells. In this study, we investigated the role of calretinin and ryanodine receptor type 1 (RyR1) gene expression in the cerebellum of tottering mice. Cerebellar tissue specimens from four experimental groups were processed for in situ hybridization histochemistry (ISHH): (1) wild-type (+/+); (2) heterozygous (tg/+) and two homozygous groups; either (3) without occurrence of an episode of paroxysmal dyskinesia (tg/tg-N); or (4) after an episode of paroxysmal dyskinesia (tg/tg-P) that lasted about 45 min on average. Quantitative analysis showed a statistically significant decrease (p = 0.0001, ANOVA) of calretinin gene expression at the level of the simple lobule of the cerebellum in both homozygous groups compared to the wild-type and heterozygous groups. RyR1 was decreased in the flocculus of the cerebellum in both the tg/tg-N and tg/tg-P groups compared to wild type (p = 0.0174, ANOVA). These results suggest that calretinin gene expression, as well as other genes involved in regulation of calcium homeostasis, such as RyR1, may play a role in the biochemical functional alterations present in tottering mice.

Amperometric analysis of exocytosis at chromaffin cells from genetically distinct mice.

Amperometry is a very powerful technique for investigating the role(s) specific proteins play in exocytosis at the single-cell level. In this study, amperometry has been used to investigate possible changes in exocytosis at chromaffin cells isolated from coloboma and tottering mutant mice. Coloboma mice possess a deletion mutation that encompasses the gene for the presynaptic protein SNAP-25 and tottering mice carry a mutation of the alpha(1A) subunit gene, which encodes the pore-forming region of P/Q-type calcium channels. Although amperometric data measured from tottering and coloboma cells are not significantly different from that measured at wild-type control cells, significant differences are found when groups of wild-type chromaffin cells are analyzed at room temperature and at 37 degrees C. Due to the large variability inherent to amperometric data, it is possible that changes in release resulting from some genetic differences cannot be detected. To fully exploit the technical advantages of using mouse chromaffin cells, experimental guidelines are described which should maximize changes in release resulting from genetic differences and increase the likelihood of detecting a change in amperometric data.