Functional properties of the purified N-type Ca2+ channel from rabbit brain

Protein partners of the calcium channel β subunit highlight new cellular functions

Calcium plays a key role in cell signalling by its intervention in a wide range of physiological processes. Its entry into cells occurs mainly via voltage-gated calcium channels (VGCC), which are found not only in the plasma membrane of excitable cells but also in cells insensitive to electrical signals. VGCC are composed of different subunits, α1, β, α2δ and γ, among which the cytosolic β subunit (Cavβ) controls the trafficking of the channel to the plasma membrane, its regulation and its gating properties. For many years, these were the main functions associated with Cavβ. However, a growing number of proteins have been found to interact with Cavβ, emphasizing the multifunctional role of this versatile protein. Interestingly, some of the newly assigned functions of Cavβ are independent of its role in the regulation of VGCC, and thus further increase its functional roles. Based on the identity of Cavβ protein partners, this review emphasizes the diverse cellular functions of Cavβ and summarizes both past findings as well as recent progress in the understanding of VGCC.

Akt regulates L-type Ca2+ channel activity by modulating Cavalpha1 protein stability

The insulin IGF-1–PI3K–Akt signaling pathway has been suggested to improve cardiac inotropism and increase Ca²⁺ handling through the effects of the protein kinase Akt. However, the underlying molecular mechanisms remain largely unknown. In this study, we provide evidence for an unanticipated regulatory function of Akt controlling L-type Ca²⁺ channel (LTCC) protein density. The pore-forming channel subunit Ca_vα1 contains highly conserved PEST sequences (signals for rapid protein degradation), and in-frame deletion of these PEST sequences results in increased Ca_vα1 protein levels. Our findings show that Akt-dependent phosphorylation of Ca_vβ2, the LTCC chaperone for Ca_vα1, antagonizes Ca_vα1 protein degradation by preventing Ca_vα1 PEST sequence recognition, leading to increased LTCC density and the consequent modulation of Ca²⁺ channel function. This novel mechanism by which Akt modulates LTCC stability could profoundly influence cardiac myocyte Ca²⁺ entry, Ca²⁺ handling, and contractility.

Unique modulation of L-type Ca2+channels by short auxiliary β1dsubunit present in cardiac muscle

Recent studies have identified a growing diversity of splice variants of auxiliary Ca2+channel Cavβ subunits. The Cavβ1disoform encodes a putative protein composed of the amino-terminal half of the full-length Cavβ1isoform and thus lacks the known high-affinity binding site that recognizes the Ca2+channel α1-subunit, the α-binding pocket. The present study investigated whether the Cavβ1dsubunit is expressed at the protein level in heart, and whether it exhibits any of the functional properties typical of full-length Cavβ subunits. On Western blots, an antibody directed against the unique carboxyl terminus of Cavβ1didentified a protein of the predicted molecular mass of 23 kDa from canine and human hearts. Immunocytochemistry and surface-membrane biotinylation experiments in transfected HEK-293 cells revealed that the full-length Cavβ1bsubunit promoted membrane trafficking of the pore-forming α1C(Cav1.2)-subunit to the surface membrane, whereas the Cavβ1dsubunit did not. Whole cell patch-clamp analysis of transfected HEK-293 cells demonstrated no effect of coexpression of the Cavβ1dwith the α1C-subunit compared with the 15-fold larger currents and leftward shift in voltage-dependent activation induced by full-length Cavβ1bcoexpression. In contrast, cell-attached patch single-channel studies demonstrated that coexpression of either Cavβ1bor Cavβ1dsignificantly increased mean open probability four- to fivefold relative to the α1C-channels alone, but only Cavβ1bcoexpression increased the number of channels observed per patch. In conclusion, the Cavβ1disoform is expressed in heart and can modulate the gating of L-type Ca2+channels, but it does not promote membrane trafficking of the channel complex.

Molecular and functional diversity of voltage-gated calcium channels

The contributing roles of voltage-gated calcium channels (VGCC) to the generation of electrical signaling are well documented. VGCCs open in response to depolarization of the plasma membrane and mediate the flux of calcium into excitable cells, which further depolarizes the membrane. But a more relevant role of VGCCs is to serve as highly regulated mechanisms to deliver calcium ions into specific intracellular locales for a variety of calcium-dependent processes including neurotransmitter release, hormone secretion, neuronal survival, and muscle contraction. Recent biochemical and molecular biological studies have demonstrated that the calcium channel pore-forming subunit (alpha 1) is not an isolated entity, but in fact interacts physically with a variety of strategically localized proteins. The functional consequences of such interactions as well as other molecular aspects of VGCC will be discussed. Finally, although far from a final conclusion, what is currently known about the molecular composition of native calcium channels will be summarized.

Differential plasma membrane targeting of voltage-dependent calcium channel subunits expressed in a polarized epithelial cell line

1. Voltage-dependent calcium channels (VDCCs) show a highly non-uniform distribution in many cell types, including neurons and other polarized secretory cells. We have examined whether this can be mimicked in a polarized epithelial cell line (Madin-Darby canine kidney), which has been used extensively to study the targeting of proteins. 2. We expressed the VDCC alpha1A, alpha1B or alpha1C subunits either alone or in combination with accessory subunits alpha2-delta and the different beta subunits, and examined their localization immunocytochemically. An alpha1 subunit was only targeted to the plasma membrane if co-expressed with the accessory subunits. 3. The combination alpha1C/alpha2-delta and all beta subunits was always localized predominantly to the basolateral membrane. It has been suggested that this is equivalent to somatodendritic targeting in neurons. 4. In contrast, the alpha1B subunit was expressed at the apical membrane with all the accessory subunit combinations, by 24 h after microinjection. This membrane destination shows some parallels with axonal targeting in neurons. 5. The alpha1A subunit was consistently observed at the apical membrane in the combinations alpha1A/alpha2-delta/beta1b or beta4. In contrast, when co-expressed with alpha2-delta/beta2a, alpha1A was clearly targeted to the basolateral membrane. 6. In conclusion, the VDCC alpha1 subunit appears to be the primary determinant for targeting the VDCC complex, but the beta subunit can modify this destination, particularly for alpha1A.

Identifying neuronal non-L Ca2+ channels--more than stamp collecting?

The pharmacology of the majority of Ca2+ channels in the nervous system is very limited. Although attempts have been made to constrain native Ca2+ channels into the framework provided by the six pore-forming molecules cloned to date, refined biophysical analysis of Ca2+ currents, expression techniques and the use of selective toxins have helped to identify unambiguously only a limited number of Ca2+ channels. In fact, many native Ca2+ channel activities remain as 'orphans', waiting for their molecular counterparts to be defined. In this article, Janet Nooney, Régis Lambert and Anne Feltz systematically delineate the well characterized non-L Ca2+ channel activities and the missing elements in our knowledge of the Ca2+ channel family.

Toxins affecting calcium channels in neurons

Calcium enters the cytoplasm mainly via voltage-activated calcium channels (VACC), and this represents a key step in the regulation of a variety of cellular processes. Advances in the fields of molecular biology, pharmacology and electrophysiology have led to the identification of several types of VACC (referred to as T-, N-, L-, P/Q- and R-types). In addition to possessing distinctive structural and functional characteristics, many of these types of calcium channels exhibit differential sensitivities to pharmacological agents. In recent years a large number of toxins, mainly small peptides, have been purified from the venom of predatory marine cone snails and spiders. Many of these toxins have specific actions on ion channels and neurotransmitter receptors, and the toxins have been used as powerful tools in neuroscience research. Some of them (omega-conotoxins, omega-agatoxins) specifically recognize and block certain types of VACC. They have common structural backbones and some been synthesized with identical potency as the natural ones. Natural, synthetic and labeled calcium channel toxins have contributed to the understanding of the diversity of the neuronal calcium channels and their function. In particular, the toxins have been useful in the study of the role of different types of calcium channels on the process of neurotransmitter release. Neuronal calcium channel toxins may develop into powerful tools for diagnosis and treatment of neurological diseases.

Structural and functional diversity of voltage-activated calcium channels

Data gathered from the expression of cDNAs that encode the subunits of voltage-dependent Ca2+ channels have demonstrated important structural and functional similarities among these channels. Despite these convergences, there are also significant differences in the nature and functional importance of subunit-subunit and protein-Ca2+ channel interactions. There is evidence demonstrating that the functional differences between Ca2+ channel subtypes is due to several factors, including the expression of distinct alpha 1 subunit proteins, the selective association of structural subunits and modulatory proteins, and differences in posttranslational processing and cell regulation. We summarize several avenues of research that should provide significant clues about the structural features involved in the biophysical and functional diversity of voltage-dependent Ca2+ channels.

Facilitation of Ca2+ current in excitable cells

Voltage-dependent Ca2+ channels are one of the main routes for the entry of Ca2+ into excitable cells. These channels are unique in cell-signalling terms in that they can transduce an electrical signal (membrane depolarization) via Ca2+ entry into a chemical signal, by virtue of the diverse range of intracellular Ca(2+)-dependent enzymes and processes. In a variety of cell types, currents through voltage-dependent Ca2+ channels can be increased in amplitude by a number of means. Although the term facilitation was originally defined as an increase of Ca2+ current resulting from one or a train of prepulses to depolarizing voltages, there is a great deal of overlap between facilitation by this means and enhancement by other routes, such as phosphorylation.

Molecular biology of calcium channels

Pharmacological and electrophysiological studies have established that there are multiple types of voltage-gated Ca2+ channels. Molecular biology has uncovered an even greater number of channel molecules. Thus, the molecular diversity of Ca2+ channels has its basis in the expression of many alpha 1 and beta genes, and also in the splice variants produced from these genes. This ability to mix and match subunits provides the cell with yet another mechanism to control the influx of calcium. Future studies will describe new subunits, the subunit composition of each type of channel, and the cloning of new Ca2+ channel types.