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Single tottering mutations responsible for the neuropathic phenotype of the P-type calcium channel
M Wakamori1, K Yamazaki, H Matsunodaira
1Department of Information Physiology, National Institute for Physiological Sciences, Okazaki, Aichi 444-8585, Japan.
Abstract:
Recent genetic and molecular biological analyses have revealed many forms of inherited channelopathies. Homozygous ataxic mice, tottering (tg) and leaner (tgla) mice, have mutations in the P/Q-type Ca2+ channel alpha1A subunit gene. Although their clinical phenotypes, histological changes, and locations of gene mutations are known, it remains unclear what phenotypes the mutant Ca2+ channels manifest, or whether the altered channel properties are the primary consequence of the mutations. To address these questions, we have characterized the electrophysiological properties of Ca2+ channels in cerebellar Purkinje cells, where the P-type is the dominant Ca2+ channel, dissociated from the normal, tg, and tgla mice, and compared them with the properties of the wild-type and mutant alpha1A channels recombinantly expressed with the alpha2 and beta subunits in baby hamster kidney cells. The most striking feature of Ca2+ channel currents of mutant Purkinje cells was a marked reduction in current density, being reduced to approximately 60 and approximately 40% of control in tg and tgla mice, respectively, without changes of cell size. The Ca2+ channel currents in the tg Purkinje cells showed a relative increase in non-inactivating component in voltage-dependent inactivation. Besides the same change, those of the tgla mice showed a more distinct change in voltage dependence of activation and inactivation, being shifted in the depolarizing direction by approximately 10 mV, with a broader voltage dependence of inactivation. In the recombinant expression system, the tg channel with a missense mutation (P601L) and one form of the two possible tgla aberrant splicing products, tgla (short) channel, showed a significant reduction in current density, while the other form of the tgla channels, tgla (long), had a current density comparable to the normal control. On the other hand, the shift in voltage dependence of activation and inactivation was observed only for the tgla (long) channel. Comparison of properties of the native and recombinant mutant channels suggests that single tottering mutations are directly responsible for the neuropathic phenotypes of reduction in current density and deviations in gating behavior, which lead to neuronal death and cerebellar atrophy.
Insights
Inherited channelopathies in tottering (tg) and leaner (tgla) mice result from P/Q-type Ca2+ channel mutations. These mutations directly cause reduced current density and altered gating, leading to neuronal death and cerebellar atrophy.
Area of Science:
- Neuroscience
- Molecular Biology
- Genetics
Background:
- Inherited channelopathies, such as those found in tottering (tg) and leaner (tgla) mice, are linked to mutations in the P/Q-type Ca2+ channel alpha1A subunit.
- The precise electrophysiological consequences of these mutations and their role in disease pathogenesis remain unclear.
Purpose of the Study:
- To electrophysiologically characterize mutant Ca2+ channels in cerebellar Purkinje cells from tg and tgla mice.
- To compare these native channels with recombinantly expressed wild-type and mutant alpha1A channels to determine the primary effects of the mutations.
Main Methods:
- Electrophysiological recordings from cerebellar Purkinje cells of normal, tg, and tgla mice.
- Recombinant expression of wild-type and mutant alpha1A subunits with alpha2 and beta subunits in baby hamster kidney cells.
- Comparison of current density, voltage-dependent activation, and inactivation properties.
Main Results:
- Mutant Purkinje cells exhibited significantly reduced Ca2+ channel current density (approx. 60% in tg, 40% in tgla).
- tgla channels showed depolarizing shifts in activation and inactivation (approx. 10 mV) and broader inactivation.
- Recombinant tg (P601L) and tgla (short) channels had reduced current density, while tgla (long) showed altered gating, not reduced density.
Conclusions:
- Mutations in the P/Q-type Ca2+ channel alpha1A subunit directly cause reduced current density and altered gating properties.
- These channel property alterations are the primary cause of neuropathic phenotypes, neuronal death, and cerebellar atrophy observed in tg and tgla mice.