Differential regulation of skeletal muscle L-type Ca2+ current and excitation-contraction coupling by the dihydropyridine receptor beta subunit

Physiological and Pathological Relevance of Selective and Nonselective Ca2+ Channels in Skeletal and Cardiac Muscle

Contraction of the striated muscle is fundamental for human existence. The action of voluntary skeletal muscle enables activities such as breathing, establishing body posture, and diverse body movements. Additionally, highly precise motion empowers communication, artistic expression, and other activities that define everyday human life. The involuntary contraction of striated muscle is the core function of the heart and is essential for blood flow. Several ion channels are important in the transduction of action potentials to cytosolic Ca2+ signals that enable muscle contraction; however, other ion channels are involved in the progression of muscle pathologies that can impair normal life or threaten it. This chapter describes types of selective and nonselective Ca2+ permeable ion channels expressed in the striated muscle, their participation in different aspects of muscle excitation and contraction, and their relevance to the progression of some pathological states.

Functional and structural characterization of a novel malignant hyperthermia-susceptible variant of DHPR-β1a subunit (CACNB1)

Malignant hyperthermia (MH) susceptibility has been recently linked to a novel variant of β_1a subunit of the dihydropyridine receptor (DHPR), a channel essential for Ca²⁺ regulation in skeletal muscle. Here we evaluate the effect of the mutant variant V156A on the structure/function of DHPR β_1a subunit and assess its role on Ca²⁺ metabolism of cultured myotubes. Using differential scanning fluorimetry, we show that mutation V156A causes a significant reduction in thermal stability of the Src homology 3/guanylate kinase core domain of β_1a subunit. Expression of the variant subunit in β₁-null mouse myotubes resulted in increased sensitivity to caffeine stimulation. Whole cell patch-clamp analysis of β_1a-V156A-expressing myotubes revealed a -2 mV shift in voltage dependence of channel activation, but no changes in Ca²⁺ conductance, current kinetics, or sarcoplasmic reticulum Ca²⁺ load were observed. Measurement of resting free Ca²⁺ and Na⁺ concentrations shows that both cations were significantly elevated in β_1a-V156A-expressing myotubes and that these changes were linked to increased rates of plasmalemmal Ca²⁺ entry through Na⁺/Ca²⁺ exchanger and/or transient receptor potential canonical channels. Overall, our data show that mutant variant V156A results in instability of protein subdomains of β_1a subunit leading to a phenotype of Ca²⁺ dysregulation that partly resembles that of other MH-linked mutations of DHPR α_1S subunit. These data prove that homozygous expression of variant β_1a-V156A has the potential to be a pathological variant, although it may require other gene defects to cause a full MH phenotype.

Stac3 has a direct role in skeletal muscle-type excitation-contraction coupling that is disrupted by a myopathy-causing mutation

In skeletal muscle, conformational coupling between CaV1.1 in the plasma membrane and type 1 ryanodine receptor (RyR1) in the sarcoplasmic reticulum (SR) is thought to underlie both excitation-contraction (EC) coupling Ca(2+) release from the SR and retrograde coupling by which RyR1 increases the magnitude of the Ca(2+) current via CaV1.1. Recent work has shown that EC coupling fails in muscle from mice and fish null for the protein Stac3 (SH3 and cysteine-rich domain 3) but did not establish the functional role of Stac3 in the CaV1.1-RyR1 interaction. We investigated this using both tsA201 cells and Stac3 KO myotubes. While confirming in tsA201 cells that Stac3 could support surface expression of CaV1.1 (coexpressed with its auxiliary β1a and α2-δ1 subunits) and the generation of large Ca(2+) currents, we found that without Stac3 the auxiliary γ1 subunit also supported membrane expression of CaV1.1/β1a/α2-δ1, but that this combination generated only tiny Ca(2+) currents. In Stac3 KO myotubes, there was reduced, but still substantial CaV1.1 in the plasma membrane. However, the CaV1.1 remaining in Stac3 KO myotubes did not generate appreciable Ca(2+) currents or EC coupling Ca(2+) release. Expression of WT Stac3 in Stac3 KO myotubes fully restored Ca(2+) currents and EC coupling Ca(2+) release, whereas expression of Stac3W280S (containing the Native American myopathy mutation) partially restored Ca(2+) currents but only marginally restored EC coupling. We conclude that membrane trafficking of CaV1.1 is facilitated by, but does not require, Stac3, and that Stac3 is directly involved in conformational coupling between CaV1.1 and RyR1.

Critical Role of Intracellular RyR1 Calcium Release Channels in Skeletal Muscle Function and Disease

The skeletal muscle Ca²⁺ release channel, also known as ryanodine receptor type 1 (RyR1), is the largest ion channel protein known and is crucial for effective skeletal muscle contractile activation. RyR1 function is controlled by Ca_v1.1, a voltage gated Ca²⁺ channel that works mainly as a voltage sensor for RyR1 activity during skeletal muscle contraction and is also fine-tuned by Ca²⁺, several intracellular compounds (e.g., ATP), and modulatory proteins (e.g., calmodulin). Dominant and recessive mutations in RyR1, as well as acquired channel alterations, are the underlying cause of various skeletal muscle diseases. The aim of this mini review is to summarize several current aspects of RyR1 function, structure, regulation, and to describe the most common diseases caused by hereditary or acquired RyR1 malfunction.

Regions of ryanodine receptors that influence activation by the dihydropyridine receptor β1a subunit

Background

Although excitation-contraction (EC) coupling in skeletal muscle relies on physical activation of the skeletal ryanodine receptor (RyR1) Ca²⁺ release channel by dihydropyridine receptors (DHPRs), the activation pathway between the DHPR and RyR1 remains unknown. However, the pathway includes the DHPR β_1a subunit which is integral to EC coupling and activates RyR1. In this manuscript, we explore the isoform specificity of β_1a activation of RyRs and the β_1a binding site on RyR1.

Methods

We used lipid bilayers to measure single channel currents and whole cell patch clamp to measure L-type Ca²⁺ currents and Ca²⁺ transients in myotubes.

Results

We demonstrate that both skeletal RyR1 and cardiac RyR2 channels in phospholipid bilayers are activated by 10–100 nM of the β_1a subunit. Activation of RyR2 by 10 nM β_1a was less than that of RyR1, suggesting a reduced affinity of RyR2 for β_1a. A reduction in activation was also observed when 10 nM β_1a was added to the alternatively spliced (ASI(−)) isoform of RyR1, which lacks ASI residues (A3481-Q3485). It is notable that the equivalent region of RyR2 also lacks four of five ASI residues, suggesting that the absence of these residues may contribute to the reduced 10 nM β_1a activation observed for both RyR2 and ASI(−)RyR1 compared to ASI(+)RyR1. We also investigated the influence of a polybasic motif (PBM) of RyR1 (K3495KKRRDGR3502) that is located immediately downstream from the ASI residues and has been implicated in EC coupling. We confirmed that neutralizing the basic residues in the PBM (RyR1 K-Q) results in an ~50 % reduction in Ca²⁺ transient amplitude following expression in RyR1-null (dyspedic) myotubes and that the PBM is also required for β_1a subunit activation of RyR1 channels in lipid bilayers. These results suggest that the removal of β_1a subunit interaction with the PBM in RyR1 could contribute directly to ~50 % of the Ca²⁺ release generated during skeletal EC coupling.

Conclusions

We conclude that the β_1a subunit likely binds to a region that is largely conserved in RyR1 and RyR2 and that this region is influenced by the presence of the ASI residues and the PBM in RyR1.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0049-3) contains supplementary material, which is available to authorized users.

Intramolecular ex vivo Fluorescence Resonance Energy Transfer (FRET) of Dihydropyridine Receptor (DHPR) β1a Subunit Reveals Conformational Change Induced by RYR1 in Mouse Skeletal Myotubes

The dihydropyridine receptor (DHPR) β_1a subunit is essential for skeletal muscle excitation-contraction coupling, but the structural organization of β_1a as part of the macromolecular DHPR-ryanodine receptor type I (RyR1) complex is still debatable. We used fluorescence resonance energy transfer (FRET) to probe proximity relationships within the β_1a subunit in cultured skeletal myotubes lacking or expressing RyR1. The fluorescein biarsenical reagent FlAsH was used as the FRET acceptor, which exhibits fluorescence upon binding to specific tetracysteine motifs, and enhanced cyan fluorescent protein (CFP) was used as the FRET donor. Ten β_1a reporter constructs were generated by inserting the CCPGCC FlAsH binding motif into five positions probing the five domains of β_1a with either carboxyl or amino terminal fused CFP. FRET efficiency was largest when CCPGCC was positioned next to CFP, and significant intramolecular FRET was observed for all constructs suggesting that in situ the β_1a subunit has a relatively compact conformation in which the carboxyl and amino termini are not extended. Comparison of the FRET efficiency in wild type to that in dyspedic (lacking RyR1) myotubes revealed that in only one construct (H458 CCPGCC β_1a -CFP) FRET efficiency was specifically altered by the presence of RyR1. The present study reveals that the C-terminal of the β_1a subunit changes conformation in the presence of RyR1 consistent with an interaction between the C-terminal of β_1a and RyR1 in resting myotubes.

β1a490-508, a 19-residue peptide from C-terminal tail of Cav1.1 β1a subunit, potentiates voltage-dependent calcium release in adult skeletal muscle fibers

The α1 and β1a subunits of the skeletal muscle calcium channel, Cav1.1, as well as the Ca(2+) release channel, ryanodine receptor (RyR1), are essential for excitation-contraction coupling. RyR1 channel activity is modulated by the β1a subunit and this effect can be mimicked by a peptide (β1a490-524) corresponding to the 35-residue C-terminal tail of the β1a subunit. Protein-protein interaction assays confirmed a high-affinity interaction between the C-terminal tail of the β1a and RyR1. Based on previous results using overlapping peptides tested on isolated RyR1, we hypothesized that a 19-amino-acid residue peptide (β1a490-508) is sufficient to reproduce activating effects of β1a490-524. Here we examined the effects of β1a490-508 on Ca(2+) release and Ca(2+) currents in adult skeletal muscle fibers subjected to voltage-clamp and on RyR1 channel activity after incorporating sarcoplasmic reticulum vesicles into lipid bilayers. β1a490-508 (25 nM) increased the peak Ca(2+) release flux by 49% in muscle fibers. Considerably fewer activating effects were observed using 6.25, 100, and 400 nM of β1a490-508 in fibers. β1a490-508 also increased RyR1 channel activity in bilayers and Cav1.1 currents in fibers. A scrambled form of β1a490-508 peptide was used as negative control and produced negligible effects on Ca(2+) release flux and RyR1 activity. Our results show that the β1a490-508 peptide contains molecular components sufficient to modulate excitation-contraction coupling in adult muscle fibers.

Three-dimensional localization of the α and β subunits and of the II-III loop in the skeletal muscle L-type Ca2+ channel

The L-type Ca(2+) channel (dihydropyridine receptor (DHPR) in skeletal muscle acts as the voltage sensor for excitation-contraction coupling. To better resolve the spatial organization of the DHPR subunits (α(1s) or Ca(V)1.1, α(2), β(1a), δ1, and γ), we created transgenic mice expressing a recombinant β(1a) subunit with YFP and a biotin acceptor domain attached to its N- and C- termini, respectively. DHPR complexes were purified from skeletal muscle, negatively stained, imaged by electron microscopy, and subjected to single-particle image analysis. The resulting 19.1-Å resolution, three-dimensional reconstruction shows a main body of 17 × 11 × 8 nm with five corners along its perimeter. Two protrusions emerge from either face of the main body: the larger one attributed to the α(2)-δ1 subunit that forms a flexible hook-shaped feature and a smaller protrusion on the opposite side that corresponds to the II-III loop of Ca(V)1.1 as revealed by antibody labeling. Novel features discernible in the electron density accommodate the atomic coordinates of a voltage-gated sodium channel and of the β subunit in a single docking possibility that defines the α1-β interaction. The β subunit appears more closely associated to the membrane than expected, which may better account for both its role in localizing the α(1s) subunit to the membrane and its suggested role in excitation-contraction coupling.

Fluorescence resonance energy transfer (FRET) indicates that association with the type I ryanodine receptor (RyR1) causes reorientation of multiple cytoplasmic domains of the dihydropyridine receptor (DHPR) α(1S) subunit

The skeletal muscle dihydropyridine receptor (DHPR) in the t-tubular membrane serves as the Ca(2+) channel and voltage sensor for excitation-contraction (EC) coupling, triggering Ca(2+) release via the type 1 ryanodine receptor (RyR1) in the sarcoplasmic reticulum (SR). The two proteins appear to be physically linked, and both the α(1S) and β(1a) subunits of the DHPR are essential for EC coupling. Within α(1S), cytoplasmic domains of importance include the I-II loop (to which β(1a) binds), the II-III and III-IV loops, and the C terminus. However, the spatial relationship of these domains to one another has not been established. Here, we have taken the approach of measuring FRET between fluorescent proteins inserted into pairs of α(1S) cytoplasmic domains. Expression of these constructs in dyspedic (RyR1 null) and dysgenic (α(1S) null) myotubes was used to test for function and targeting to plasma membrane/SR junctions and to test whether the presence of RyR1 caused altered FRET. We found that in the absence of RyR1, measureable FRET occurred between the N terminus and C terminus (residue 1636), and between the II-III loop (residue 626) and both the N and C termini; the I-II loop (residue 406) showed weak FRET with the II-III loop but not with the N terminus. Association with RyR1 caused II-III loop FRET to decrease with the C terminus and increase with the N terminus and caused I-II loop FRET to increase with both the II-III loop and N terminus. Overall, RyR1 appears to cause a substantial reorientation of the cytoplasmic α(1S) domains consistent with their becoming more closely packed.

Bimolecular fluorescence complementation and targeted biotinylation provide insight into the topology of the skeletal muscle Ca ( 2+) channel β1a subunit

In skeletal muscle, L-type calcium channels (DHPRs), localized to plasma membrane sarcoplasmic reticulum junctions, are tightly packed into groups of four termed tetrads. Here, we have used bimolecular fluorescence complementation (BiFC) and targeted biotinylation to probe the structure and organization of β1a subunits associated with native CaV 1.1 in DHPRs of myotubes. The construct YN-β1a-YC, in which the non-fluorescent fragments of YFP ("YN" corresponding to YFP residues 1-158, and "YC" corresponding to YFP residues 159-238) were fused, respectively, to the N- and C-termini of β1a, was fully functional and displayed yellow fluorescence within DHPR tetrads after expression in β1-knockout (β1KO) myotubes; this yellow fluorescence demonstrated the occurrence of BiFC of YN and YC on the β1a N- and C-termini. In these experiments, we avoided overexpression because control experiments in non-muscle cells indicated that this could result in non-specific BiFC. BiFC of YN-β1a-YC in DHPR tetrads appeared to be intramolecular between N- and C-termini of individual β1a subunits rather than between adjacent DHPRs because BiFC (1) was observed for YN-β1a-YC co-expressed with CaV 1.2 (which does not form tetrads) and (2) was not observed after co-expression of YN-β1a-YN plus YC-β1a-YC in β1KO myotubes. Thus, β1a function is compatible with N- and C-termini being close enough together to allow BiFC. However, both termini appeared to have positional freedom and not to be closely opposed by other junctional proteins since both were accessible to gold-streptavidin conjugates. Based on these results, a model is proposed for the arrangement of β1a subunits in DHPR tetrads.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

Proper restoration of excitation-contraction coupling in the dihydropyridine receptor beta1-null zebrafish relaxed is an exclusive function of the beta1a subunit

The paralyzed zebrafish strain relaxed carries a null mutation for the skeletal muscle dihydropyridine receptor (DHPR) beta(1a) subunit. Lack of beta(1a) results in (i) reduced membrane expression of the pore forming DHPR alpha(1S) subunit, (ii) elimination of alpha(1S) charge movement, and (iii) impediment of arrangement of the DHPRs in groups of four (tetrads) opposing the ryanodine receptor (RyR1), a structural prerequisite for skeletal muscle-type excitation-contraction (EC) coupling. In this study we used relaxed larvae and isolated myotubes as expression systems to discriminate specific functions of beta(1a) from rather general functions of beta isoforms. Zebrafish and mammalian beta(1a) subunits quantitatively restored alpha(1S) triad targeting and charge movement as well as intracellular Ca(2+) release, allowed arrangement of DHPRs in tetrads, and most strikingly recovered a fully motile phenotype in relaxed larvae. Interestingly, the cardiac/neuronal beta(2a) as the phylogenetically closest, and the ancestral housefly beta(M) as the most distant isoform to beta(1a) also completely recovered alpha(1S) triad expression and charge movement. However, both revealed drastically impaired intracellular Ca(2+) transients and very limited tetrad formation compared with beta(1a). Consequently, larval motility was either only partially restored (beta(2a)-injected larvae) or not restored at all (beta(M)). Thus, our results indicate that triad expression and facilitation of 1,4-dihydropyridine receptor (DHPR) charge movement are common features of all tested beta subunits, whereas the efficient arrangement of DHPRs in tetrads and thus intact DHPR-RyR1 coupling is only promoted by the beta(1a) isoform. Consequently, we postulate a model that presents beta(1a) as an allosteric modifier of alpha(1S) conformation enabling skeletal muscle-type EC coupling.

New Determinant for the CaVbeta2 subunit modulation of the CaV1.2 calcium channel

Ca(v)beta subunits support voltage gating of Ca(v)1.2 calcium channels and play important role in excitation-contraction coupling. The common central membrane-associated guanylate kinase (MAGUK) region of Ca(v)beta binds to the alpha-interaction domain (AID) and the IQ motif of the pore-forming alpha(1C) subunit, but these two interactions do not explain why the cardiac Ca(v)beta(2) subunit splice variants differentially modulate inactivation of Ca(2+) currents (I(Ca)). Previously we described beta(2Deltag), a functionally active splice variant of human Ca(v)beta(2) lacking MAGUK. By deletion analysis of beta(2Deltag), we have now identified a 41-amino acid C-terminal essential determinant (beta(2)CED) that stimulates I(Ca) in the absence of Ca(v)beta subunits and conveys a +20-mV shift in the peak of the I(Ca)-voltage relationship. The beta(2)CED is targeted by alpha(1C) to the plasma membrane, forms a complex with alpha(1C) but does not bind to AID. Electrophysiology and binding studies point to the calmodulin-interacting LA/IQ region in the alpha(1C) subunit C terminus as a functionally relevant beta(2)CED binding site. The beta(2)CED interacts with LA/IQ in a Ca(2+)- and calmodulin-independent manner and need LA, but not IQ, to activate the channel. Deletion/mutation analyses indicated that each of the three Ca(v)beta(2)/alpha(1C) interactions is sufficient to support I(Ca). However, beta(2)CED does not support Ca(2+)-dependent inactivation, suggesting that interactions of MAGUK with AID and IQ are crucial for Ca(2+)-induced inactivation. The beta(2)CED is conserved only in Ca(v)beta(2) subunits. Thus, beta(2)CED constitutes a previously unknown integrative part of the multifactorial mechanism of Ca(v)beta(2)-subunit differential modulation of the Ca(v)1.2 calcium channel that in beta(2Deltag) occurs without MAGUK.

Alpha2delta1 dihydropyridine receptor subunit is a critical element for excitation-coupled calcium entry but not for formation of tetrads in skeletal myotubes

It has been shown that small interfering RNA (siRNA) partial knockdown of the alpha(2)delta(1) dihydropyridine receptor subunits cause a significant increase in the rate of activation of the L-type Ca(2+) current in myotubes but have little or no effect on skeletal excitation-contraction coupling. This study used permanent siRNA knockdown of alpha(2)delta(1) to address two important unaddressed questions. First, does the alpha(2)delta(1) subunit contribute to the size and/or spacing of tetradic particles? Second, is the alpha(2)delta(1) subunit important for excitation-coupled calcium entry? We found that the size and spacing of tetradic particles is unaffected by siRNA knockdown of alpha(2)delta(1), indicating that the visible particle represents the alpha(1s) subunit. Strikingly, >97% knockdown of alpha(2)delta(1) leads to a complete loss of excitation-coupled calcium entry during KCl depolarization and a more rapid decay of Ca(2+) transients during bouts of repetitive electrical stimulation like those occurring during normal muscle activation in vivo. Thus, we conclude that the alpha(2)delta(1) dihydropyridine receptor subunit is physiologically necessary for sustaining Ca(2+) transients in response to prolonged depolarization or repeated trains of action potentials.

The role of L- and T-type Ca2+ currents during the in vitro aging of murine myogenic (i28) cells in culture

The age-related decline in skeletal muscle strength could, in part, result from alterations in the mechanism of excitation-contraction coupling, responsible for muscle contraction. In the present work, we used the in vitro aging of murine myogenic (i28) cells as a model, to investigate whether the inefficiency of aged satellite cells to generate functional skeletal muscle fibres could be partly due to defective voltage-dependent Ca2+ currents. The whole-cell patch clamp technique was employed to measure L- and T-type Ca2+ currents in myotubes derived from the differentiation and fusion of these cells reaching replicative senescence. Our data showed that the expression and the amplitude of these currents decreased significantly during in vitro aging. Moreover, the analysis of the L-type current evoked in young and old cells by positive voltage steps, revealed no differences in the kinetics of activation, but significant alterations in the rate of inactivation. These effects of in vitro aging on voltage-dependent Ca2+ currents could also be related to their inability to fuse into myotubes. Taken together, our data support the hypothesis that age-related effects on voltage-dependent L- and T-type currents could be one of the causes of the failure of satellite cells to efficiently counteract the impairment in muscle force.

The role of voltage-gated calcium channels in pancreatic beta-cell physiology and pathophysiology

Voltage-gated calcium (CaV) channels are ubiquitously expressed in various cell types throughout the body. In principle, the molecular identity, biophysical profile, and pharmacological property of CaV channels are independent of the cell type where they reside, whereas these channels execute unique functions in different cell types, such as muscle contraction, neurotransmitter release, and hormone secretion. At least six CaValpha1 subunits, including CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1, have been identified in pancreatic beta-cells. These pore-forming subunits complex with certain auxiliary subunits to conduct L-, P/Q-, N-, R-, and T-type CaV currents, respectively. beta-Cell CaV channels take center stage in insulin secretion and play an important role in beta-cell physiology and pathophysiology. CaV3 channels become expressed in diabetes-prone mouse beta-cells. Point mutation in the human CaV1.2 gene results in excessive insulin secretion. Trinucleotide expansion in the human CaV1.3 and CaV2.1 gene is revealed in a subgroup of patients with type 2 diabetes. beta-Cell CaV channels are regulated by a wide range of mechanisms, either shared by other cell types or specific to beta-cells, to always guarantee a satisfactory concentration of Ca2+. Inappropriate regulation of beta-cell CaV channels causes beta-cell dysfunction and even death manifested in both type 1 and type 2 diabetes. This review summarizes current knowledge of CaV channels in beta-cell physiology and pathophysiology.

Inositol-1,4,5-triphosphate receptors mediate activity-induced synaptic Ca2+ signals in muscle fibers and Ca2+ overload in slow-channel syndrome

Strict control of calcium entry through excitatory synaptic receptors is important for shaping synaptic responses, gene expression, and cell survival. Disruption of this control may lead to pathological accumulation of Ca2+. The slow-channel congenital myasthenic syndrome (SCS), due to mutations in muscle acetylcholine receptor (AChR), perturbs the kinetics of synaptic currents, leading to post-synaptic Ca2+ accumulation. To understand the regulation of calcium signaling at the neuromuscular junction (NMJ) and the etiology of Ca2+ overload in SCS we studied the role of sarcoplasmic Ca2+ stores in SCS. Using fura-2 loaded dissociated fibers activated with acetylcholine puffs, we confirmed that Ca2+ accumulates around wild type NMJ and discovered that Ca2+ accumulates significantly faster around the NMJ of SCS transgenic dissociated muscle fibers. Additionally, we determined that this process is dependant on the activation, altered kinetics, and movement of Ca2+ ions through the AChR, although, surprisingly, depletion of intracellular stores also prevents the accumulation of this cation around the NMJ. Finally, we concluded that the sarcoplasmic reticulum is the main source of Ca2+ and that inositol-1,4,5-triphosphate receptors (IP3R), and to a lesser degree L-type voltage gated Ca2+ channels, are responsible for the efflux of this cation from intracellular stores. These results suggest that a signaling system mediated by the activation of AChR, Ca2+, and IP3R is responsible for localized Ca2+ signals observed in muscle fibers and the Ca2+ overload observed in SCS.

Calcium Homeostasis in Human Placenta: Role of Calcium‐Handling Proteins

The human placenta is a transitory organ, representing during pregnancy the unique connection between the mother and her fetus. The syncytiotrophoblast represents the specialized unit in the placenta that is directly involved in fetal nutrition, mainly involving essential nutrients, such as lipids, amino acids, and calcium. This ion is of particular interest since it is actively transported by the placenta throughout pregnancy and is associated with many roles during intrauterine life. At term, the human fetus has accumulated about 25-30 g of calcium. This transfer allows adequate fetal growth and development, since calcium is vital for fetal skeleton mineralization and many cellular functions, such as signal transduction, neurotransmitter release, and cellular growth. Thus, there are many proteins involved in calcium homeostasis in the human placenta.

Sarcoplasmic reticulum: The dynamic calcium governor of muscle

The sarcoplasmic reticulum (SR) provides feedback control required to balance the processes of calcium storage, release, and reuptake in skeletal muscle. This balance is achieved through the concerted action of three major classes of SR calcium-regulatory proteins: (1) luminal calcium-binding proteins (calsequestrin, histidine-rich calcium-binding protein, junctate, and sarcalumenin) for calcium storage; (2) SR calcium release channels (type 1 ryanodine receptor or RyR1 and IP3 receptors) for calcium release; and (3) sarco(endo)plasmic reticulum Ca2+ -ATPase (SERCA) pumps for calcium reuptake. Proper calcium storage, release, and reuptake are essential for normal skeletal muscle function. We review SR structure and function during normal skeletal muscle activity, the proteins that orchestrate calcium storage, release, and reuptake, and how phenotypically distinct muscle diseases (e.g., malignant hyperthermia, central core disease, and Brody disease) can result from subtle alterations in the activity of several key components of the SR calcium-regulatory machinery.

Organization of calcium channel beta1a subunits in triad junctions in skeletal muscle

In skeletal muscle, dihydropyridine receptors (DHPRs) in the plasma membrane interact with the type 1 ryanodine receptor (RyR1) at junctions with the sarcoplasmic reticulum. This interaction organizes junctional DHPRs into groups of four termed tetrads. In addition to the principle alpha1S subunit, the beta1a subunit of the DHPR is also important for the interaction with RyR1. To probe this interaction, we measured fluorescence resonance energy transfer (FRET) of beta1a subunits labeled with cyan fluorescent protein (CFP) and/or yellow fluorescent protein (YFP). Expressed in dysgenic (alpha1S-null) myotubes, YFP-beta1a-CFP and CFP-beta1a-YFP were diffusely distributed in the cytoplasm and highly mobile as indicated by fluorescence recovery after photobleaching. Thus, beta1a does not appear to bind to other cellular proteins in the absence of alpha1S. FRET efficiencies for these cytoplasmic beta1a subunits were approximately 6-7%, consistent with the idea that <10 nm separates the N and C termini. After coexpression with unlabeled alpha1S (in dysgenic or beta1-null myotubes), both constructs produced discrete fluorescent puncta, which correspond to assembled DHPRs in junctions and that did not recover after photobleaching. In beta1-null myotubes, FRET efficiencies of doubly labeled beta1a in puncta were similar to those of the same constructs diffusely distributed in the cytoplasm and appeared to arise intramolecularly, since no FRET was measured when mixtures of singly labeled beta1a (CFP or YFP at the N or C terminus) were expressed in beta1-null myotubes. Thus, DHPRs in tetrads may be arranged such that the N and C termini of adjacent beta1a subunits are located >10 nm from one another.

The beta 1a subunit is essential for the assembly of dihydropyridine-receptor arrays in skeletal muscle

Homozygous zebrafish of the mutant relaxed (red(ts25)) are paralyzed and die within days after hatching. A significant reduction of intramembrane charge movements and the lack of depolarization-induced but not caffeine-induced Ca(2+) transients suggested a defect in the skeletal muscle dihydropyridine receptor (DHPR). Sequencing of DHPR cDNAs indicated that the alpha(1S) subunit is normal, whereas the beta(1a) subunit harbors a single point mutation resulting in a premature stop. Quantitative RT-PCR revealed that the mutated gene is transcribed, but Western blot analysis and immunocytochemistry demonstrated the complete loss of the beta(1a) protein in mutant muscle. Thus, the immotile zebrafish relaxed is a beta(1a)-null mutant. Interestingly, immunocytochemistry showed correct triad targeting of the alpha(1S) subunit in the absence of beta(1a). Freeze-fracture analysis of the DHPR clusters in relaxed myotubes revealed an approximately 2-fold reduction in cluster size with a normal density of DHPR particles within the clusters. Most importantly, DHPR particles in the junctional membranes of the immotile zebrafish mutant relaxed entirely lacked the normal arrangement in arrays of tetrads. Thus, our data indicate that the lack of the beta(1a) subunit does not prevent triad targeting of the DHPR alpha(1S) subunit but precludes the skeletal muscle-specific arrangement of DHPR particles opposite the ryanodine receptor (RyR1). This defect properly explains the complete deficiency of skeletal muscle excitation-contraction coupling in beta(1)-null model organisms.

Short-term regulation of excitation-contraction coupling by the beta1a subunit in adult mouse skeletal muscle

The beta1a subunit of the skeletal muscle voltage-gated Ca2+ channel plays a fundamental role in the targeting of the channel to the tubular system as well as in channel function. To determine whether this cytosolic auxiliary subunit is also a regulatory protein of Ca2+ release from the sarcoplasmic reticulum in vivo, we pressure-injected the beta1a subunit into intact adult mouse muscle fibers and recorded, with Fluo-3 AM, the intracellular Ca2+ signal induced by the action potential. We found that the beta1a subunit significantly increased, within minutes, the amplitude of Ca2+ release without major changes in its time course. beta1a subunits with the carboxy-terminus region deleted did not show an effect on Ca2+ release. The possibility that potentiation of Ca2+ release is due to a direct interaction between the beta1a subunit and the ryanodine receptor was ruled out by bilayer experiments of RyR1 single-channel currents and also by Ca2+ flux experiments. Our data suggest that the beta1a subunit is capable of regulating E-C coupling in the short term and that the integrity of the carboxy-terminus region is essential for its modulatory effect.

Involvement of a heptad repeat in the carboxyl terminus of the dihydropyridine receptor beta1a subunit in the mechanism of excitation-contraction coupling in skeletal muscle

Chimeras consisting of the homologous skeletal dihydropyridine receptor (DHPR) beta1a subunit and the heterologous cardiac/brain beta2a subunit were used to determine which regions of beta1a were responsible for the skeletal-type excitation-contraction (EC) coupling phenotype. Chimeras were transiently transfected in beta1 knockout myotubes and then voltage-clamped with simultaneous measurement of confocal fluo-4 fluorescence. All chimeras expressed a similar density of DHPR charge movements, indicating that the membrane density of DHPR voltage sensors was not a confounding factor in these studies. The data indicates that a beta1a-specific domain present in the carboxyl terminus, namely the D5 region comprising the last 47 residues (beta1a 478-524), is essential for expression of skeletal-type EC coupling. Furthermore, the location of beta1aD5 immediately downstream from conserved domain D4 is also critical. In contrast, chimeras in which beta1aD5 was swapped by the D5 region of beta2a expressed Ca(2+) transients triggered by the Ca(2+) current, or none at all. A hydrophobic heptad repeat is present in domain D5 of beta1a (L478, V485, V492). To determine the role of this motif, residues in the heptad repeat were mutated to alanines. The triple mutant beta1a(L478A/V485A/V492A) recovered weak skeletal-type EC coupling (DeltaF/F(max) = 0.4 +/- 0.1 vs. 2.7 +/- 0.5 for wild-type beta1a). However, a triple mutant with alanine substitutions at positions out of phase with the heptad repeat, beta1a(S481A/L488A/S495A), was normal (DeltaF/F(max) = 2.1 +/- 0.4). In summary, the presence of the beta1a-specific D5 domain, in its correct position after conserved domain D4, is essential for skeletal-type EC coupling. Furthermore, a heptad repeat in beta1aD5 controls the EC coupling activity. The carboxyl terminal heptad repeat of beta1a might be involved in protein-protein interactions with ryanodine receptor type 1 required for DHPR to ryanodine receptor type 1 signal transmission.

Functional interaction of CaV channel isoforms with ryanodine receptors studied in dysgenic myotubes

The L-type Ca(2+) channels Ca(V)1.1 (alpha(1S)) and Ca(V)1.2 (alpha(1C)) share properties of targeting but differ by their mode of coupling to ryanodine receptors in muscle cells. The brain isoform Ca(V)2.1 (alpha(1A)) lacks ryanodine receptor targeting. We studied these three isoforms in myotubes of the alpha(1S)-deficient skeletal muscle cell line GLT under voltage-clamp conditions and estimated the flux of Ca(2+) (Ca(2+) input flux) resulting from Ca(2+) entry and release. Surprisingly, amplitude and kinetics of the input flux were similar for alpha(1C) and alpha(1A) despite a previously reported strong difference in responsiveness to extracellular stimulation. The kinetic flux characteristics of alpha(1C) and alpha(1A) resembled those in alpha(1S)-expressing cells but the contribution of Ca(2+) entry was much larger. alpha(1C) but not alpha(1A)-expressing cells revealed a distinct transient flux component sensitive to sarcoplasmic reticulum depletion by 30 microM cyclopiazonic acid and 10 mM caffeine. This component likely results from synchronized Ca(2+)-induced Ca(2+) release that is absent in alpha(1A)-expressing myotubes. In cells expressing an alpha(1A)-derivative (alpha(1)Aas(1592-clip)) containing the putative targeting sequence of alpha(1S), a similar transient component was noticeable. Yet, it was considerably smaller than in alpha(1C), indicating that the local Ca(2+) entry produced by the chimera is less effective in triggering Ca(2+) release despite similar global Ca(2+) inward current density.

Targeting mechanisms of high voltage-activated Ca2+ channels

Functional voltage-dependent Ca2+ channel complexes are assembled by three to four subunits: alpha1, beta, alpha2delta subunits (C. Leveque et al., 1994, J. Biol Chem. 269, 6306-6312; M. W. McEnery et al., 1991, Proc. Natl. Acad. Sci. U.S.A. 88, 11095-11099) and at least in muscle cells also y subunits (B. M. Curtis and W. A. Catterall, 1984, Biochemistry 23, 2113-2118). Ca2+ channels mediate the voltage-dependent Ca2+ influx in subcellular compartments, triggering such diverse processes as neurotransmitter release, dendritic action potentials, excitation-contraction, and excitation-transcription coupling. The targeting of biophysically defined Ca2+ channel complexes to the correct subcellular structures is, thus, critical to proper cell and physiological functioning. Despite their importance, surprisingly little is known about the targeting mechanisms by which Ca2+ channel complexes are transported to their site of function. Here we summarize what we know about the targeting of Ca2+ channel complexes through the cell to the plasma membrane and subcellular structures.

Ca2+-dependent excitation-contraction coupling triggered by the heterologous cardiac/brain DHPR beta2a-subunit in skeletal myotubes

Molecular determinants essential for skeletal-type excitation-contraction (EC) coupling have been described in the cytosolic loops of the dihydropyridine receptor (DHPR) alpha1S pore subunit and in the carboxyl terminus of the skeletal-specific DHPR beta1a-subunit. It is unknown whether EC coupling domains present in the beta-subunit influence those present in the pore subunit or if they act independent of each other. To address this question, we investigated the EC coupling signal that is generated when the endogenous DHPR pore subunit alpha1S is paired with the heterologous heart/brain DHPR beta2a-subunit. Studies were conducted in primary cultured myotubes from beta1 knockout (KO), ryanodine receptor type 1 (RyR1) KO, ryanodine receptor type 3 (RyR3) KO, and double RyR1/RyR3 KO mice under voltage clamp with simultaneous monitoring of confocal fluo-4 fluorescence. The beta2a-mediated Ca2+ current recovered in beta1 KO myotubes lacking the endogenous DHPR beta1a-subunit verified formation of the alpha1S/beta1a pair. In myotube genotypes which express no or low-density L-type Ca2+ currents, namely beta1 KO and RyR1 KO, beta2a overexpression recovered a wild-type density of nifedipine-sensitive Ca2+ currents with a slow activation kinetics typical of skeletal myotubes. Concurrent with Ca2+ current recovery, there was a drastic reduction of voltage-dependent, skeletal-type EC coupling and emergence of Ca2+ transients triggered by the Ca2+ current. A comparison of beta2a overexpression in RyR3 KO, RyR1 KO, and double RyR1/RyR3 KO myotubes concluded that both RyR1 and RyR3 isoforms participated in Ca2+-dependent Ca2+ release triggered by the beta2a-subunit. In beta1 KO and RyR1 KO myotubes, the Ca2+-dependent EC coupling promoted by beta2a overexpression had the following characteristics: 1), L-type Ca2+ currents had a wild-type density; 2), Ca2+ transients activated much slower than controls overexpressing beta1a, and the rate of fluorescence increase was consistent with the activation kinetics of the Ca2+ current; 3), the voltage dependence of the Ca2+ transient was bell-shaped and the maximum was centered at approximately +30 mV, consistent with the voltage dependence of the Ca2+ current; and 4), Ca2+ currents and Ca2+ transients were fully blocked by nifedipine. The loss in voltage-dependent EC coupling promoted by beta2a was inferred by the drastic reduction in maximal Ca2+ fluorescence at large positive potentials (DeltaF/Fmax) in double dysgenic/beta1 KO myotubes overexpressing the pore mutant alpha1S (E1014K) and beta2a. The data indicate that beta2a, upon interaction with the skeletal pore subunit alpha1S, overrides critical EC coupling determinants present in alpha1S. We propose that the alpha1S/beta pair, and not the alpha1S-subunit alone, controls the EC coupling signal in skeletal muscle.

Auxiliary subunits: essential components of the voltage-gated calcium channel complex

Voltage-gated calcium channels are important mediators of several physiological processes, including neuronal excitability and muscle contraction. At the molecular level, the channels are composed of four subunits--the pore forming alpha(1) subunit and the auxiliary alpha(2)delta, beta and gamma subunits. The auxiliary subunits modulate the trafficking and the biophysical properties of the alpha(1) subunit. In the past several years there has been an acceleration of our understanding of the auxiliary subunits, primarily because of their molecular characterization and the availability of spontaneous and targeted mouse mutants. These studies have revealed the crucial role of the subunits in the functional effects that are mediated by voltage-gated calcium channels.

The random-coil 'C' fragment of the dihydropyridine receptor II-III loop can activate or inhibit native skeletal ryanodine receptors

The actions of peptide C, corresponding to (724)Glu-Pro(760) of the II-III loop of the skeletal dihydropyridine receptor, on ryanodine receptor (RyR) channels incorporated into lipid bilayers with the native sarcoplasmic reticulum membrane show that the peptide is a high-affinity activator of native skeletal RyRs at cytoplasmic concentrations of 100 nM-10 microM. In addition, we found that peptide C inhibits RyRs in a voltage-independent manner when added for longer times or at higher concentrations (up to 150 microM). Peptide C had a random-coil structure indicating that it briefly assumes a variety of structures, some of which might activate and others which might inhibit RyRs. The results suggest that RyR activation and inhibition by peptide C arise from independent stochastic processes. A rate constant of 7.5 x 10(5) s(-1).M(-1) was obtained for activation and a lower estimate for the rate constant for inhibition of 5.9 x 10(3) s(-1).M(-1). The combined actions of peptide C and peptide A (II-III loop sequence (671)Thr-Leu(690)) showed that peptide C prevented activation but not blockage of RyRs by peptide A. We suggest that the effects of peptide C indicate functional interactions between a part of the dihydropyridine receptor and the RyR. These interactions could reflect either dynamic changes that occur during excitation-contraction coupling or interactions between the proteins at rest.

Ca2+ current and charge movements in skeletal myotubes promoted by the beta-subunit of the dihydropyridine receptor in the absence of ryanodine receptor type 1

The beta-subunit of the dihydropyridine receptor (DHPR) enhances the Ca(2+) channel and voltage-sensing functions of the DHPR. In skeletal myotubes, there is additional modulation of DHPR functions imposed by the presence of ryanodine receptor type-1 (RyR1). Here, we examined the participation of the beta-subunit in the expression of L-type Ca(2+) current and charge movements in RyR1 knock-out (KO), beta1 KO, and double beta1/RyR1 KO myotubes generated by mating heterozygous beta1 KO and RyR1 KO mice. Primary myotube cultures of each genotype were transfected with various beta-isoforms and then whole-cell voltage-clamped for measurements of Ca(2+) and gating currents. Overexpression of the endogenous skeletal beta1a isoform resulted in a low-density Ca(2+) current either in RyR1 KO (36 +/- 9 pS/pF) or in beta1/RyR1 KO (34 +/- 7 pS/pF) myotubes. However, the heterologous beta2a variant with a double cysteine motif in the N-terminus (C3, C4), recovered a Ca(2+) current that was entirely wild-type in density in RyR1 KO (195 +/- 16 pS/pF) and was significantly enhanced in double beta1/RyR1 KO (115 +/- 18 pS/pF) myotubes. Other variants tested from the four beta gene families (beta1a, beta1b, beta1c, beta3, and beta4) were unable to enhance Ca(2+) current expression in RyR1 KO myotubes. In contrast, intramembrane charge movements in beta2a-expressing beta1a/RyR1 KO myotubes were significantly lower than in beta1a-expressing beta1a/RyR1 KO myotubes, and the same tendency was observed in the RyR1 KO myotube. Thus, beta2a had a preferential ability to recover Ca(2+) current, whereas beta1a had a preferential ability to rescue charge movements. Elimination of the double cysteine motif (beta2a C3,4S) eliminated the RyR1-independent Ca(2+) current expression. Furthermore, Ca(2+) current enhancement was observed with a beta2a variant lacking the double cysteine motif and fused to the surface membrane glycoprotein CD8. Thus, tethering the beta2a variant to the myotube surface activated the DHPR Ca(2+) current and bypassed the requirement for RyR1. The data suggest that the Ca(2+) current expressed by the native skeletal DHPR complex has an inherently low density due to inhibitory interactions within the DHPR and that the beta1a-subunit is critically involved in process.

Truncation of the carboxyl terminus of the dihydropyridine receptor beta1a subunit promotes Ca2+ dependent excitation-contraction coupling in skeletal myotubes

We investigated the contribution of the carboxyl terminus region of the beta1a subunit of the skeletal dihydropyridine receptor (DHPR) to the mechanism of excitation-contraction (EC) coupling. cDNA-transfected beta1 KO myotubes were voltage clamped, and Ca(2+) transients were analyzed by confocal fluo-4 fluorescence. A chimera with an amino terminus half of beta2a and a carboxyl terminus half of beta1a (beta2a 1-287/beta1a 325-524) recapitulates skeletal-type EC coupling quantitatively and was used to generate truncated variants lacking 7 to 60 residues from the beta1a-specific carboxyl terminus (Delta7, Delta21, Delta29, Delta35, and Delta60). Ca(2+) transients recovered by the control chimera have a sigmoidal Ca(2+) fluorescence (DeltaF/F) versus voltage curve with saturation at potentials more positive than +30 mV, independent of external Ca(2+) and stimulus duration. In contrast, the amplitude of Ca(2+) transients expressed by the truncated variants varied with the duration of the pulse, and for Delta29, Delta35, and Delta60, also varied with external Ca(2+) concentration. For Delta7 and Delta21, a 50-ms depolarization produced a sigmoidal DeltaF/F versus voltage curve with a lower than control maximum fluorescence. Moreover, for Delta29, Delta35, and Delta60, a 200-ms depolarization increased the maximum fluorescence and changed the shape of the DeltaF/F versus voltage curve, from sigmoidal to bell-shaped, with a maximum at approximately +30 mV. The change in voltage dependence, together with the external Ca(2+) dependence and additional controls with ryanodine, indicated a loss of skeletal-type EC coupling and the emergence of an EC coupling component triggered by the Ca(2+) current. Analyses of d(DeltaF/F)/dt showed that the rate of cytosolic Ca(2+) increase during the Ca(2+) transient was fivefold faster for the control chimera than for the severely truncated variants (Delta29, Delta35, and Delta60) and was consistent with the kinetics of the DHPR Ca(2+) current. In summary, absence of the beta1a-specific carboxyl terminus (last 29 to 60 residues of the control chimera) results in a loss of the fast component of the Ca(2+) transient, bending of the DeltaF/F versus voltage curve, and emergence of EC coupling triggered by the Ca(2+) current. The studies underscore the essential role of the carboxyl terminus region of the DHPR beta1a subunit in fast voltage dependent EC coupling in skeletal myotubes.

The beta1a subunit regulates the functional properties of adult frog and mouse L-type Ca2+ channels of skeletal muscle

The beta(1a) subunit, one of the auxiliary subunits of Ca(V)1.1 channels, was expressed in COS-1 cells, purified by electroelution and electrodialysis techniques and identified by Western blot using monoclonal antibodies. The purified beta(1a) subunit strongly interacted in vitro with the alpha interaction domain (AID) of Ca(V)1.1 channels. The actions of the purified beta(1a) subunit on Ca(V)1.1 channel currents were assessed in whole cell voltage clamp experiments performed in vesicles derived from frog and mouse adult skeletal muscle plasma membranes. L-type inward currents were recorded in solutions containing Ba(2+) (I(Ba)). Values of peak I(Ba) were doubled by the beta(1a) subunit in frog and mouse muscle vesicles and the amplitude of the slow component of tail currents was greatly increased. The actions of the beta(1a) subunit on Ca(V)1.1 channel currents reached a steady state within 20 min. The beta(1a) subunit had no effect on the time courses of activation or inactivation of I(Ba) or shifted the current-voltage relation. Non-linear capacitive currents were recorded in solutions that contained mostly impermeant ions. Charge movement depended on voltage with average Boltzmann parameters: Q(max) = 28.0 +/- 6.6 nC microF(-1), V = -58.0 +/- 2.0 mV and k = 15.3 +/- 1.1 mV (n = 24). In the presence of the beta(1a) subunit, these parameters remained unchanged: Q(max) = 29.8 +/- 3.5 nC microF(-1), V = -54.5 +/- 2.2 mV and k = 16.4 +/- 1.3 mV (n = 21). Overall, the work describes a novel preparation to explore in situ the role of the beta(1a) subunit on the function of adult Ca(V)1.1 channels.

Novel functional properties of Ca(2+) channel beta subunits revealed by their expression in adult rat heart cells

Recombinant adenoviruses were used to overexpress green fluorescent protein (GFP)-fused auxiliary Ca(2+) channel beta subunits (beta(1)-beta(4)) in cultured adult rat heart cells, to explore new dimensions of beta subunit functions in vivo. Distinct beta-GFP subunits distributed differentially between the surface sarcolemma, transverse elements, and nucleus in single heart cells. All beta-GFP subunits increased the native cardiac whole-cell L-type Ca(2+) channel current density, but produced distinctive effects on channel inactivation kinetics. The degree of enhancement of whole-cell current density was non-uniform between beta subunits, with a rank order of potency beta(2a) approximately equal to beta(4) > beta(1b) > beta(3). For each beta subunit, the increase in L-type current density was accompanied by a correlative increase in the maximal gating charge (Q(max)) moved with depolarization. However, beta subunits produced characteristic effects on single L-type channel gating, resulting in divergent effects on channel open probability (P(o)). Quantitative analysis and modelling of single-channel data provided a kinetic signature for each channel type. Spurred on by ambiguities regarding the molecular identity of the actual endogenous cardiac L-type channel beta subunit, we cloned a new rat beta(2) splice variant, beta(2b), from heart using 5' rapid amplification of cDNA ends (RACE) PCR. By contrast with beta(2a), expression of beta(2b) in heart cells yielded channels with a microscopic gating signature virtually identical to that of native unmodified channels. Our results provide novel insights into beta subunit functions that are unattainable in traditional heterologous expression studies, and also provide new perspectives on the molecular identity of the beta subunit component of cardiac L-type Ca(2+) channels. Overall, the work establishes a powerful experimental paradigm to explore novel functions of ion channel subunits in their native environments.

Mice with cardiac-specific sequestration of the beta-subunit of the L-type calcium channel

The beta subunit of the L-type voltage-dependent calcium channel modifies the properties of the channel complex by both allosteric modulation of the alpha1 subunit function and by chaperoning the translocation of the alpha1 subunit to the plasma membrane. The goal of this study was to investigate the functional effect of changing the in vivo stoichiometry between the alpha1 and beta subunits by creating a dominant negative expression system in a transgenic mouse model. The high affinity beta subunit-binding domain of the alpha1 subunit was overexpressed in a cardiac-specific manner to act as a beta subunit trap. We found that the predominant beta isoform was located primarily in the membrane bound fraction of heart protein, whereas the beta1 and beta3 were mostly cytosolic. There was a significant diminution of the amount of beta2 in the membrane fraction of the transgenic animals, resulting in a decrease in contractility of the heart and a decrease in L-type calcium current density in the myocyte. However, there were no distinguishable differences in beta1 and beta3 protein expression levels in the membrane bound fraction between transgenic and non-transgenic animals. Since the beta1 and beta3 isoforms only make up a small portion of the total beta subunit in the heart, slight changes in this fraction are not detectable using Western analysis. In contrast, beta1 and beta3 in skeletal muscle and brain, the predominant isoforms in these tissues, respectively, are membrane bound.

Interactions between dihydropyridine receptors and ryanodine receptors in striated muscle

Excitation-contraction coupling in both skeletal and cardiac muscle depends on structural and functional interactions between the voltage-sensing dihydropyridine receptor L-type Ca(2+) channels in the surface/transverse tubular membrane and ryanodine receptor Ca(2+) release channels in the sarcoplasmic reticulum membrane. The channels are targeted to either side of a narrow junctional gap that separates the external and internal membrane systems and are arranged so that bi-directional structural and functional coupling can occur between the proteins. There is strong evidence for a physical interaction between the two types of channel protein in skeletal muscle. This evidence is derived from studies of excitation-contraction coupling in intact myocytes and from experiments in isolated systems where fragments of the dihydropyridine receptor can bind to the ryanodine receptors in sarcoplasmic reticulum vesicles or in lipid bilayers and alter channel activity. Although micro-regions that participate in the functional interactions have been identified in each protein, the role of these regions and the molecular nature of the protein-protein interaction remain unknown. The trigger for Ca(2+) release through ryanodine receptors in cardiac muscle is a Ca(2+) influx through the L-type Ca(2+) channel. The Ca(2+) entering through the surface membrane Ca(2+) channels flows directly onto underlying ryanodine receptors and activates the channels. This was thought to be a relatively simple system compared with that in skeletal muscle. However, complexities are emerging and evidence has now been obtained for a bi-directional physical coupling between the proteins in cardiac as well as skeletal muscle. The molecular nature of this coupling remains to be elucidated.

Functional analysis of a frame-shift mutant of the dihydropyridine receptor pore subunit (alpha1S) expressing two complementary protein fragments

BACKGROUND

The L-type Ca2+ channel formed by the dihydropyridine receptor (DHPR) of skeletal muscle senses the membrane voltage and opens the ryanodine receptor (RyR1). This channel-to-channel coupling is essential for Ca2+ signaling but poorly understood. We characterized a single-base frame-shift mutant of alpha1S, the pore subunit of the DHPR, that has the unusual ability to function voltage sensor for excitation-contraction (EC) coupling by virtue of expressing two complementary hemi-Ca2+ channel fragments.

RESULTS

Functional analysis of cDNA transfected dysgenic myotubes lacking alpha1S were carried out using voltage-clamp, confocal Ca2+ indicator fluoresence, epitope immunofluorescence and immunoblots of expressed proteins. The frame-shift mutant (fs-alpha1S) expressed the N-terminal half of alpha1S (M1 to L670) and the C-terminal half starting at M701 separately. The C-terminal fragment was generated by an unexpected restart of translation of the fs-alpha1S message at M701 and was eliminated by a M701I mutation. Protein-protein complementation between the two fragments produced recovery of skeletal-type EC coupling but not L-type Ca2+ current.

DISCUSSION

A premature stop codon in the II-III loop may not necessarily cause a loss of DHPR function due to a restart of translation within the II-III loop, presumably by a mechanism involving leaky ribosomal scanning. In these cases, function is recovered by expression of complementary protein fragments from the same cDNA. DHPR-RyR1 interactions can be achieved via protein-protein complementation between hemi-Ca2+ channel proteins, hence an intact II-III loop is not essential for coupling the DHPR voltage sensor to the opening of RyR1 channel.

A component of excitation-contraction coupling triggered in the absence of the T671-L690 and L720-Q765 regions of the II-III loop of the dihydropyridine receptor alpha(1s) pore subunit

We conducted a deletion analysis of two regions identified in the II-III loop of alpha(1S), residues 671-690, which were shown to bind to ryanodine receptor type 1 (RyR1) and stimulate RyR1 channels in vitro, and residues 720-765 or the narrower 724-743 region, which confer excitation-contraction (EC) coupling function to chimeric dihydropyridine receptors (DHPRs). Deletion mutants were expressed in dysgenic alpha(1S)-null myotubes and analyzed by voltage-clamp and confocal fluo-4 fluorescence. Immunostaining of the mutant subunits using an N-terminus tag revealed abundant protein expression in all cases. Furthermore, the maximum recovered charge movement density was >80% of that recovered by full-length alpha(1S) in all cases. Delta671-690 had no effect on the magnitude of voltage-evoked Ca(2+) transients or the L-type Ca(2+) current density. In contrast, Delta720-765 or Delta724-743 abolished Ca(2+) transients entirely, and L-type Ca(2+) current was reduced or absent. Surprisingly, Ca(2+) transients and Ca(2+) currents of a moderate magnitude were recovered by the double deletion mutant Delta671-690/Delta720-765. A simple explanation for this result is that Delta720-765 induces a conformation change that disrupts EC coupling, and this conformational change is partially reverted by Delta671-690. To test for Ca(2+)-entry independent EC coupling, a pore mutation (E1014K) known to entirely abolish the inward Ca(2+) current was introduced. alpha(1S) Delta671-690/Delta720-765/E1014K expressed Ca(2+) transients with Boltzmann parameters identical to those of the Ca(2+)-conducting double deletion construct. The data strongly suggest that skeletal-type EC coupling is not uniquely controlled by alpha(1S) 720-765. Other regions of alpha(1S) or other DHPR subunits must therefore directly contribute to the activation of RyR1 during EC coupling.

Modulation of L-type Ca2+ current but not activation of Ca2+ release by the gamma1 subunit of the dihydropyridine receptor of skeletal muscle

BACKGROUND

The multisubunit (alpha1S,alpha2-delta, beta1a and gamma1) skeletal muscle dihydropyridine receptor (DHPR) transduces membrane depolarization into release of Ca2+ from the sarcoplasmic reticulum (SR) and also acts as an L-type Ca2+ channel. To more fully investigate the function of the gamma1 subunit in these two processes, we produced mice lacking this subunit by gene targeting.

RESULTS

Mice lacking the DHPR gamma1 subunit (gamma1 null) survive to adulthood, are fertile and have no obvious gross phenotypic abnormalities. The gamma1 subunit is expressed at approximately half the normal level in heterozygous mice (gamma1 het). The density of the L-type Ca2+ current in gamma1 null and gamma1 het myotubes was higher than in controls. Inactivation of the Ca2+ current produced by a long depolarization was slower and incomplete in gamma1 null and gamma1 het myotubes, and was shifted to a more positive potential than in controls. However, the half-activation potential of intramembrane charge movements was not shifted, and the maximum density of the total charge was unchanged. Also, no shift was observed in the voltage-dependence of Ca2+ transients. gamma1 null and gamma1 het myotubes had the same peak Ca2+ amplitude vs. voltage relationship as control myotubes.

CONCLUSIONS

The L-type Ca2+ channel function, but not the SR Ca2+ release triggering function of the skeletal muscle dihydropyridine receptor, is modulated by the gamma1 subunit.

Intramembrane charge movements and excitation- contraction coupling expressed by two-domain fragments of the Ca2+ channel

To investigate the molecular basis of the voltage sensor that triggers excitation-contraction (EC) coupling, the four-domain pore subunit of the dihydropyridine receptor (DHPR) was cut in the cytoplasmic linker between domains II and III. cDNAs for the I-II domain (alpha1S 1-670) and the III-IV domain (alpha1S 701-1873) were expressed in dysgenic alpha1S-null myotubes. Coexpression of the two fragments resulted in complete recovery of DHPR intramembrane charge movement and voltage-evoked Ca(2+) transients. When fragments were expressed separately, EC coupling was not recovered. However, charge movement was detected in the I-II domain expressed alone. Compared with I-II and III-IV together, the charge movement in the I-II domain accounted for about half of the total charge (Q(max) = 3 +/- 0.23 vs. 5.4 +/- 0.76 fC/pF, respectively), and the half-activation potential for charge movement was significantly more negative (V(1/2) = 0.2 +/- 3.5 vs. 22 +/- 3.4 mV, respectively). Thus, interactions between the four internal domains of the pore subunit in the assembled DHPR profoundly affect the voltage dependence of intramembrane charge movement. We also tested a two-domain I-II construct of the neuronal alpha1A Ca(2+) channel. The neuronal I-II domain recovered charge movements like those of the skeletal I-II domain but could not assist the skeletal III-IV domain in the recovery of EC coupling. The results demonstrate that a functional voltage sensor capable of triggering EC coupling in skeletal myotubes can be recovered by the expression of complementary fragments of the DHPR pore subunit. Furthermore, the intrinsic voltage-sensing properties of the alpha1A I-II domain suggest that this hemi-Ca(2+) channel could be relevant to neuronal function.

Functional impact of the ryanodine receptor on the skeletal muscle L-type Ca(2+) channel

L-type Ca(2+) channel (L-channel) activity of the skeletal muscle dihydropyridine receptor is markedly enhanced by the skeletal muscle isoform of the ryanodine receptor (RyR1) (Nakai, J., R.T. Dirksen, H. T. Nguyen, I.N. Pessah, K.G. Beam, and P.D. Allen. 1996. Nature. 380:72-75.). However, the dependence of the biophysical and pharmacological properties of skeletal L-current on RyR1 has yet to be fully elucidated. Thus, we have evaluated the influence of RyR1 on the properties of macroscopic L-currents and intracellular charge movements in cultured skeletal myotubes derived from normal and "RyR1-knockout" (dyspedic) mice. Compared with normal myotubes, dyspedic myotubes exhibited a 40% reduction in the amount of maximal immobilization-resistant charge movement (Q(max), 7.5 +/- 0.8 and 4.5 +/- 0.4 nC/muF for normal and dyspedic myotubes, respectively) and an approximately fivefold reduction in the ratio of maximal L-channel conductance to charge movement (G(max)/Q(max)). Thus, RyR1 enhances both the expression level and Ca(2+) conducting activity of the skeletal L-channel. For both normal and dyspedic myotubes, the sum of two exponentials was required to fit L-current activation and resulted in extraction of the amplitudes (A(fast) and A(slow)) and time constants (tau(slow) and tau(fast)) for each component of the macroscopic current. In spite of a >10-fold in difference current density, L-currents in normal and dyspedic myotubes exhibited similar relative contributions of fast and slow components (at +40 mV; A(fast)/[A(fast) + A(slow)] approximately 0.25). However, both tau(fast) and tau(slow) were significantly (P < 0.02) faster for myotubes lacking the RyR1 protein (tau(fast), 8.5 +/- 1.2 and 4.4 +/- 0.5 ms; tau(slow), 79.5 +/- 10.5 and 34.6 +/- 3.7 ms at +40 mV for normal and dyspedic myotubes, respectively). In both normal and dyspedic myotubes, (-) Bay K 8644 (5 microM) caused a hyperpolarizing shift (approximately 10 mV) in the voltage dependence of channel activation and an 80% increase in peak L-current. However, the increase in peak L-current correlated with moderate increases in both A(slow) and A(fast) in normal myotubes, but a large increase in only A(fast) in dyspedic myotubes. Equimolar substitution of Ba(2+) for extracellular Ca(2+) increased both A(fast) and A(slow) in normal myotubes. The identical substitution in dyspedic myotubes failed to significantly alter the magnitude of either A(fast) or A(slow). These results demonstrate that RyR1 influences essential properties of skeletal L-channels (expression level, activation kinetics, modulation by dihydropyridine agonist, and divalent conductance) and supports the notion that RyR1 acts as an important allosteric modulator of the skeletal L-channel, analogous to that of a Ca(2+) channel accessory subunit.

Involvement of the carboxy-terminus region of the dihydropyridine receptor beta1a subunit in excitation-contraction coupling of skeletal muscle

Skeletal muscle knockout cells lacking the beta subunit of the dihydropyridine receptor (DHPR) are devoid of slow L-type Ca(2+) current, charge movements, and excitation-contraction coupling, despite having a normal Ca(2+) storage capacity and Ca(2+) spark activity. In this study we identified a specific region of the missing beta1a subunit critical for the recovery of excitation-contraction. Experiments were performed in beta1-null myotubes expressing deletion mutants of the skeletal muscle-specific beta1a, the cardiac/brain-specific beta2a, or beta2a/beta1a chimeras. Immunostaining was used to determine that all beta constructs were expressed in these cells. We examined the Ca(2+) conductance, charge movements, and Ca(2+) transients measured by confocal fluo-3 fluorescence of transfected myotubes under whole-cell voltage-clamp. All constructs recovered an L-type Ca(2+) current with a density, voltage-dependence, and kinetics of activation similar to that recovered by full-length beta1a. In addition, all constructs except beta2a mutants recovered charge movements with a density similar to full-length beta1a. Thus, all beta constructs became integrated into a skeletal-type DHPR and, except for beta2a mutants, all restored functional DHPRs to the cell surface at a high density. The maximum amplitude of the Ca(2+) transient was not affected by separate deletions of the N-terminus of beta1a or the central linker region of beta1a connecting two highly conserved domains. Also, replacement of the N-terminus half of beta1a with that of beta2a had no effect. However, deletion of 35 residues of beta1a at the C-terminus produced a fivefold reduction in the maximum amplitude of the Ca(2+) transients. A similar observation was made by deletion of the C-terminus of a chimera in which the C-terminus half was from beta1a. The identified domain at the C-terminus of beta1a may be responsible for colocalization of DHPRs and ryanodine receptors (RyRs), or may be required for the signal that opens the RyRs during excitation-contraction coupling. This new role of DHPR beta in excitation-contraction coupling represents a cell-specific function that could not be predicted on the basis of functional expression studies in heterologous cells.