cAMP-dependent protein kinase rapidly phosphorylates serine- 687 of the skeletal muscle receptor for calcium channel blockers

β subunits of voltage-gated calcium channels in cardiovascular diseases

Calcium signaling is required in bodily functions essential for survival, such as muscle contractions and neuronal communications. Of note, the voltage-gated calcium channels (VGCCs) expressed on muscle and neuronal cells, as well as some endocrine cells, are transmembrane protein complexes that allow for the selective entry of calcium ions into the cells. The α1 subunit constitutes the main pore-forming subunit that opens in response to membrane depolarization, and its biophysical functions are regulated by various auxiliary subunits-β, α2δ, and γ subunits. Within the cardiovascular system, the γ-subunit is not expressed and is therefore not discussed in this review. Because the α1 subunit is the pore-forming subunit, it is a prominent druggable target and the focus of many studies investigating potential therapeutic interventions for cardiovascular diseases. While this may be true, it should be noted that the direct inhibition of the α1 subunit may result in limited long-term cardiovascular benefits coupled with undesirable side effects, and that its expression and biophysical properties may depend largely on its auxiliary subunits. Indeed, the α2δ subunit has been reported to be essential for the membrane trafficking and expression of the α1 subunit. Furthermore, the β subunit not only prevents proteasomal degradation of the α1 subunit, but also directly modulates the biophysical properties of the α1 subunit, such as its voltage-dependent activities and open probabilities. More importantly, various isoforms of the β subunit have been found to differentially modulate the α1 subunit, and post-translational modifications of the β subunits further add to this complexity. These data suggest the possibility of the β subunit as a therapeutic target in cardiovascular diseases. However, emerging studies have reported the presence of cardiomyocyte membrane α1 subunit trafficking and expression in a β subunit-independent manner, which would undermine the efficacy of β subunit-targeting drugs. Nevertheless, a better understanding of the auxiliary β subunit would provide a more holistic approach when targeting the calcium channel complexes in treating cardiovascular diseases. Therefore, this review focuses on the post-translational modifications of the β subunit, as well as its role as an auxiliary subunit in modulating the calcium channel complexes.

Pigment Translocation in Caridean Shrimp Chromatophores: Receptor Type, Signal Transduction, Second Messengers, and Cross Talk Among Multiple Signaling Cascades

Pigment aggregation in shrimp chromatophores is triggered by red pigment concentrating hormone (RPCH), a neurosecretory peptide whose plasma membrane receptor may be a G-protein coupled receptor (GPCR). While RPCH binding activates the Ca²⁺ /cGMP signaling cascades, a role for cyclic AMP (cAMP) in pigment aggregation is obscure, as are the steps governing Ca²⁺ release from the smooth endoplasmic reticulum (SER). A role for the antagonistic neuropeptide, pigment dispersing homone (α-PDH) is also unclear. In red, ovarian chromatophores from the freshwater shrimp Macrobrachium olfersi, we show that a G-protein antagonist (AntPG) strongly inhibits RPCH-triggered pigment aggregation, suggesting that RPCH binds to a GPCR, activating an inhibitory G-protein. Decreasing cAMP levels may cue pigment aggregation, since cytosolic cAMP titers, when augmented by cholera toxin, forskolin or vinpocentine, completely or partially impair pigment aggregation. Triggering opposing Ca²⁺ /cGMP and cAMP cascades by simultaneous perfusion with lipid-soluble cyclic nucleotide analogs induces a "tug-of-war" response, pigments aggregating in some chromatosomes with unpredictable, oscillatory movements in others. Inhibition of cAMP-dependent protein kinase accelerates aggregation and reduces dispersion velocities, suggesting a role in phosphorylation events, possibly regulating SER Ca²⁺ release and pigment aggregation. The second messengers IP₃ and cADPR do not stimulate SER Ca²⁺ release. α-PDH does not sustain pigment dispersion, suggesting that pigment translocation in caridean chromatophores may be regulated solely by RPCH, since PDH is not required. We propose a working hypothesis to further unravel key steps in the mechanisms of pigment translocation within crustacean chromatophores that have remained obscure for nearly a century.

Conformation of dehydropentapeptides containing four achiral amino acid residues - controlling the role of L-valine

Structural studies of pentapeptides containing an achiral block, built from two dehydroamino acid residues (Δ^ZPhe and ΔAla) and two glycines, as well as one chiral L-Val residue were performed using NMR spectroscopy. The key role of the L-Val residue in the generation of the secondary structure of peptides is discussed. The obtained results suggest that the strongest influence on the conformation of peptides arises from a valine residue inserted at the C-terminal position. The most ordered conformation was found for peptide Boc-Gly-ΔAla-Gly-Δ^ZPhe-Val-OMe (3), which adopts a right-handed helical conformation.

Supramolecular assemblies and localized regulation of voltage-gated ion channels

This review addresses the localized regulation of voltage-gated ion channels by phosphorylation. Comprehensive data on channel regulation by associated protein kinases, phosphatases, and related regulatory proteins are mainly available for voltage-gated Ca2+ channels, which form the main focus of this review. Other voltage-gated ion channels and especially Kv7.1-3 (KCNQ1-3), the large- and small-conductance Ca2+-activated K+ channels BK and SK2, and the inward-rectifying K+ channels Kir3 have also been studied to quite some extent and will be included. Regulation of the L-type Ca2+ channel Cav1.2 by PKA has been studied most thoroughly as it underlies the cardiac fight-or-flight response. A prototypical Cav1.2 signaling complex containing the beta2 adrenergic receptor, the heterotrimeric G protein Gs, adenylyl cyclase, and PKA has been identified that supports highly localized via cAMP. The type 2 ryanodine receptor as well as AMPA- and NMDA-type glutamate receptors are in close proximity to Cav1.2 in cardiomyocytes and neurons, respectively, yet independently anchor PKA, CaMKII, and the serine/threonine phosphatases PP1, PP2A, and PP2B, as is discussed in detail. Descriptions of the structural and functional aspects of the interactions of PKA, PKC, CaMKII, Src, and various phosphatases with Cav1.2 will include comparisons with analogous interactions with other channels such as the ryanodine receptor or ionotropic glutamate receptors. Regulation of Na+ and K+ channel phosphorylation complexes will be discussed in separate papers. This review is thus intended for readers interested in ion channel regulation or in localization of kinases, phosphatases, and their upstream regulators.

NovelN-acyl dehydroalanine derivatives as antioxidants: Studies on rat liver lipid peroxidation levels and DPPH free radical scavenging activity

Oxidative stress has been implicated in the development of many neurodegenerative diseases such as Parkinson and Alzhemier's disease and is also responsible for aging, artherosclerosis, rheumatoid arthritis and carcinogenesis. Olefins such as dehydroalanines have been shown to inactivate free radicals by forming stabilized free radical adducts. Among these molecules N-acyl dehydroalanines react with and scavenge oxygen and hydroxyl radicals. This study describes the synthesis, characterization and in vitro effects on rat liver lipid peroxidation levels, and DPPH free radical scavenging activities of some N-acyl dehydroalanine derivatives. Compounds c, f and j slightly scavenged the level of DPPH radical at 10(-3) M concentration by about 27, 46, and 56%, respectively while compounds a, d, e, f, g, h showed a strong inhibitory effect on lipid peroxidation at 10(-3)M and 10(-4)M concentrations and inhibition was in the range of 76-90%. The possible antioxidant mechanism of the compounds was discussed.

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.

Autoinhibitory control of the CaV1.2 channel by its proteolytically processed distal C-terminal domain

Voltage-gated Ca(2+) channels of the Ca(V)1 family initiate excitation-contraction coupling in cardiac, smooth, and skeletal muscle and are primary targets for regulation by the sympathetic nervous system in the 'fight-or-flight' response. In the heart, activation of beta-adrenergic receptors greatly increases the L-type Ca(2+) current through Ca(V)1.2 channels, which requires phosphorylation by cyclic AMP-dependent protein kinase (PKA) anchored via an A-kinase anchoring protein (AKAP15). Surprisingly, the site of interaction of PKA and AKAP15 lies in the distal C-terminus, which is cleaved from the remainder of the channel by in vivo proteolytic processing. Here we report that the proteolytically cleaved distal C-terminal domain forms a specific molecular complex with the truncated alpha(1) subunit and serves as a potent autoinhibitory domain. Formation of the autoinhibitory complex greatly reduces the coupling efficiency of voltage sensing to channel opening and shifts the voltage dependence of activation to more positive membrane potentials. Ab initio structural modelling and site-directed mutagenesis revealed a binding interaction between a pair of arginine residues in a predicted alpha-helix in the proximal C-terminal domain and a set of three negatively charged amino acid residues in a predicted helix-loop-helix bundle in the distal C-terminal domain. Disruption of this interaction by mutation abolished the inhibitory effects of the distal C-terminus on Ca(V)1.2 channel function. These results provide the first functional characterization of this autoinhibitory complex, which may be a major form of the Ca(V)1 family Ca(2+) channels in cardiac and skeletal muscle cells, and reveal a unique ion channel regulatory mechanism in which proteolytic processing produces a more effective autoinhibitor of Ca(V)1.2 channel function.

Molecular regulation of voltage-gated Ca2+ channels

Voltage-gated Ca2+ (Ca(v)) channels are found in all excitable cells and many nonexcitable cells, in which they govern Ca2+ influx, thereby contributing to determine a host of important physiological processes including gene transcription, muscle contraction, hormone secretion, and neurotransmitter release. The past years have seen some significant advances in our understanding of the functional, pharmacological, and molecular properties of Ca(v) channels. Molecular studies have revealed that several of these channels are oligomeric complexes consisting of an ion-conducting alpha1 subunit and auxiliary alpha2delta, beta, and gamma subunits. In addition, cloning of multiple Ca(v) channel alpha1 subunits has offered the opportunity to investigate the regulation of these proteins at the molecular level. The regulation of Ca(v) channels by intracellular second messengers constitutes a key mechanism for controlling Ca2+ influx. This review summarizes recent advances that have provided important clues to the underlying molecular mechanisms involved in the regulation of Ca(v) channels by protein phosphorylation, G-protein activation, and interactions with Ca(2+)-binding and SNARE proteins.

Activation of mGluR5 modulates Ca2+ currents in retinal amacrine cells from the chick

In the inner plexiform layer, amacrine cells receive glutamatergic input from bipolar cells. Glutamate can depolarize amacrine cells by activation of ionotropic glutamate receptors or mediate potentially more diverse changes via activation of G protein-coupled metabotropic glutamate receptors (mGluR5). Here, we asked whether selective activation of metabotropic glutamate receptor 5 is linked to modulation of the voltage-gated Ca2+ channels expressed by cultured GABAergic amacrine cells. To address this, we performed whole-cell voltage clamp experiments, primarily in the perforated-patch configuration. We found that agonists selective for mGluR5, including (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), enhanced the amplitude of the voltage-dependent Ca2+ current. The voltage-dependent Ca2+ current and CHPG-dependent current enhancement were blocked by nifedipine, indicating that L-type Ca2+ channels, specifically, were being modulated. We have previously shown that activation of mGluR5 produces Ca2+ elevations in cultured amacrine cells (Sosa et al., 2002). Loading the cells with 5 mM BAPTA inhibited the mGluR5-dependent enhancement, suggesting that the cytosolic Ca2+ elevations are required for modulation of the current. Although activation of mGluR5 is typically linked to activation of protein kinase C, we found that direct activation of this kinase leads to inhibition of the Ca2+ current, indicating that stimulation of this enzyme is not responsible for the mGluR5-dependent enhancement. Interestingly, direct stimulation of protein kinase A produced an enhancement of the Ca2+ current similar to that observed with activation of mGluR5. Thus, activation of mGluR5 may modulate the L-type voltage-gated Ca2+ current in these GABAergic amacrine cells via activation of protein kinase A, possibly via direct activation of a Ca2(+)-dependent adenylate cyclase.

Convergent regulation of skeletal muscle Ca2+ channels by dystrophin, the actin cytoskeleton, and cAMP-dependent protein kinase

The skeletal muscle L-type Ca2+ channel (Ca(V)1.1), which is responsible for initiating muscle contraction, is regulated by phosphorylation by cAMP-dependent protein kinase (PKA) in a voltage-dependent manner that requires direct physical association between the channel and the kinase mediated through A-kinase anchoring proteins (AKAPs). The role of the actin cytoskeleton in channel regulation was investigated in skeletal myocytes cultured from wild-type mice, mdx mice that lack the cytoskeletal linkage protein dystrophin, and a skeletal muscle cell line, 129 CB3. Voltage dependence of channel activation was shifted positively, and potentiation was greatly diminished in mdx myocytes and in 129 CB3 cells treated with the microfilament stabilizer phalloidin. Voltage-dependent potentiation by strong depolarizing prepulses was reduced in mdx myocytes but could be restored by positively shifting the stimulus potentials to compensate for the positive shift in the voltage dependence of gating. Inclusion of PKA in the pipette caused a negative shift in the voltage dependence of activation and restored voltage-dependent potentiation in mdx myocytes. These results show that skeletal muscle Ca2+ channel activity and voltage-dependent potentiation are controlled by PKA and microfilaments in a convergent manner. Regulation of Ca2+ channel activity by hormones and neurotransmitters that use the PKA signal transduction pathway may interact in a critical way with the cytoskeleton and may be impaired by deletion of dystrophin, contributing to abnormal regulation of intracellular calcium concentrations in dystrophic muscle.

Removal of clustered positive charge from dihydropyridine receptor II-III loop peptide augments activation of ryanodine receptors

Peptides based on the skeletal muscle DHPR II-III loop have been shown to regulate ryanodine receptor channel activity. The N-terminal region of this cytoplasmic loop is predicted to adopt an alpha-helical conformation. We have selected a peptide sequence of 26 residues (Ala(667)-Asp(692)) as the minimum sequence to emulate the helical propensity of the corresponding protein sequence. The interaction of this control peptide with skeletal and cardiac RyR channels in planar lipid bilayers was then assessed and was found to lack isoform specificity. At low concentrations peptide A(667)-D(692) increased RyR open probability, whilst at higher concentrations open probability was reduced. By replacing a region of clustered positive charge with a neutral sequence with the same predisposition to helicity, the inhibitory effect was ablated and activation was enhanced. This novel finding demonstrates that activation does not derive from the presence of positively charged residues adjacent in the primary structure and, although it may be mediated by the alignment of basic residues down one face of an amphipathic helix, not all of these residues are essential.

Voltage-gated Ca2+ channel Ca(V)1.3 subunit expressed in the hair cell epithelium of the sacculus of the trout Oncorhynchus mykiss: cloning and comparison across vertebrate classes

Full-length sequence (>6.5 kb) has been determined for the Ca(V)1.3 pore-forming subunit of the voltage-gated Ca(2+) channel from the saccular hair cells of the rainbow trout (Oncorhynchus mykiss). Primary structure was obtained from overlapping PCR and cloned fragments, amplified by primers based on teleost, avian, and mammalian sources. Trout saccular Ca(V)1.3 was localized to hair cells, as evidenced by its isolation from an epithelial layer in which the hair cell is the only intact cell type. The predicted amino acid sequence of the trout hair cell Ca(V)1.3 is approximately 70% identical to the sequences of avian and mammalian Ca(V)1.3 subunits and shows L-type characteristics. The trout hair cell Ca(V)1.3 expresses a 26-aa insert in the I-II cytoplasmic loop (exon 9a) and a 10-aa insert in the IVS2-IVS3 cytoplasmic loop (exon 30a), neither of which is appreciably represented in trout brain. The exon 9a insert also occurs in hair cell organs of chick and rat, and appears as an exon in human genomic Ca(V)1.3 sequence (but not in the Ca(V)1.3 coding sequence expressed in human brain or pancreas). The exon 30a insert, although expressed in hair cells of chick as well as trout, does not appear in comparable rat or human tissues. Further, the IIIS2 region shows a splice choice (exon 22a) that is associated with the hair cell organs of trout, chick, and rat, but is not found in human genomic sequence. The elucidation of the primary structure of the voltage-gated Ca(2+) channel Ca(V)1.3 subunit from hair cells of the teleost, representing the lowest of the vertebrate classes, suggests a generality of sensory mechanism for Ca(V)1.3 across hair cell systems. In particular, the exon 9a insert of this channel appears to be the molecular feature most consistently associated with hair cells from fish to mammal, consonant with the hypothesis that the latter region may be a signature for the hair cell.

Skeletal muscle L-type Ca(2+) current modulation in gamma1-deficient and wildtype murine myotubes by the gamma1 subunit and cAMP

Modulation of the steady-state inactivation and current amplitude by the gamma1 subunit of the murine skeletal muscle L-type Ca(2+) channel were investigated using the whole-cell patch-clamp technique. Transient expression of the gamma1 subunit, but not of the gamma2 (stargazin) protein, in primary cultured myotubes from gamma1-deficient mice shifted the steady-state inactivation approximately -15 mV, thereby restoring wildtype (WT) steady-state inactivation and current amplitude. The increased Ca(2+) current amplitude in gamma1-deficient cells was abolished in myotubes from animals of 4 weeks and older whereas the positive shift in steady-state inactivation was independent of mouse age. Raising intracellular cAMP levels using the membrane-permeant analogue 8-Br-cAMP led to an increase in Ca(2+) current amplitude in WT cells to the level in gamma1-deficient myotubes. There was no effect on the current amplitude in gamma1-deficient cells or on the steady-state inactivation in either genotype. Rp-cAMPS, a competitive inhibitor of cAMP-dependent protein kinase, had no effect on the WT Ca(2+) current amplitude and steady-state inactivation, but diminished the current amplitude in gamma1-deficient myotubes without affecting the steady-state inactivation in these cells. These data show that the increased Ca(2+) influx in myotubes lacking the gamma1 subunit, due to right-shifted steady-state inactivation and increased L-type Ca(2+) current amplitude, is determined by the gamma1 subunit. The effect on current amplitude depends on the age of the mice and its cAMP-dependent modulation appears to be controlled by the gamma1 subunit.

Asymmetric arrangement of auxiliary subunits of skeletal muscle voltage-gated l-type Ca(2+) channel

Highly purified L-type Ca(2+) channel complexes containing all five subunits (alpha(1), alpha(2), beta, gamma, and delta) and complexes of alpha(1)-beta subunits were obtained from skeletal muscle triad membranes by three-step purification and by 1% Triton X-100 treatment, respectively. Their structures and the subunit arrangements were analyzed by electron microscopy. Projection images of negatively stained Ca(2+) channels and alpha(1)-beta complexes were aligned, classified and averaged. The alpha(1)-beta complex showed a hollow trapezoid shape of 12 nm height. In top view, four asymmetric domains surrounded a central depression predicted to form the channel pore. The complete Ca(2+) channel complex exhibited the cylindrical shape of 20 nm in height binding a spherical domain on one edge. Further image analysis of higher complexes of the Ca(2+) channel using a monoclonal antibody against the beta subunit showed that the alpha(1)-beta complex forms the non-decorated side of the cylinder, which can traverse the membrane from outside the cell to the cytoplasm. Based on these results, we propose that the Ca(2+) channel exhibits an asymmetric arrangement of auxiliary subunits.

Structure and regulation of voltage-gated Ca2+ channels

Voltage-gated Ca(2+) channels mediate Ca(2+) entry into cells in response to membrane depolarization. Electrophysiological studies reveal different Ca(2+) currents designated L-, N-, P-, Q-, R-, and T-type. The high-voltage-activated Ca(2+) channels that have been characterized biochemically are complexes of a pore-forming alpha1 subunit of approximately 190-250 kDa; a transmembrane, disulfide-linked complex of alpha2 and delta subunits; an intracellular beta subunit; and in some cases a transmembrane gamma subunit. Ten alpha1 subunits, four alpha2delta complexes, four beta subunits, and two gamma subunits are known. The Cav1 family of alpha1 subunits conduct L-type Ca(2+) currents, which initiate muscle contraction, endocrine secretion, and gene transcription, and are regulated primarily by second messenger-activated protein phosphorylation pathways. The Cav2 family of alpha1 subunits conduct N-type, P/Q-type, and R-type Ca(2+) currents, which initiate rapid synaptic transmission and are regulated primarily by direct interaction with G proteins and SNARE proteins and secondarily by protein phosphorylation. The Cav3 family of alpha1 subunits conduct T-type Ca(2+) currents, which are activated and inactivated more rapidly and at more negative membrane potentials than other Ca(2+) current types. The distinct structures and patterns of regulation of these three families of Ca(2+) channels provide a flexible array of Ca(2+) entry pathways in response to changes in membrane potential and a range of possibilities for regulation of Ca(2+) entry by second messenger pathways and interacting proteins.

Effects of dihydropyridine receptor II-III loop peptides on Ca(2+) release in skinned skeletal muscle fibers

In skeletal muscle fibers, the intracellular loop between domains II and III of the alpha(1)-subunit of the dihydropyridine receptor (DHPR) may directly activate the adjacent Ca(2+) release channel in the sarcoplasmic reticulum. We examined the effects of synthetic peptide segments of this loop on Ca(2+) release in mechanically skinned skeletal muscle fibers with functional excitation-contraction coupling. In rat fibers at physiological Mg(2+) concentration ([Mg(2+)]; 1 mM), a 20-residue skeletal muscle DHPR peptide [A(S(20)); Thr(671)-Leu(690); 30 microM], shown previously to induce Ca(2+) release in a triad preparation, caused only small spontaneous force responses in approximately 40% of fibers, although it potentiated responses to depolarization and caffeine in all fibers. The COOH-terminal half of A(S(20)) [A(S(10))] induced much larger spontaneous responses but also caused substantial inhibition of Ca(2+) release to both depolarization and caffeine. Both peptides induced or potentiated Ca(2+) release even when the voltage sensors were inactivated, indicating direct action on the Ca(2+) release channels. The corresponding 20-residue cardiac DHPR peptide [A(C(20)); Thr(793)-Ala(812)] was ineffective, but its COOH-terminal half [A(C(10))] had effects similar to A(S(20)). In the presence of lower [Mg(2+)] (0.2 mM), exposure to either A(S(20)) or A(C(10)) (30 microM) induced substantial Ca(2+) release. Peptide C(S) (100 microM), a loop segment reported to inhibit Ca(2+) release in triads, caused partial inhibition of depolarization-induced Ca(2+) release. In toad fibers, each of the A peptides had effects similar to or greater than those in rat fibers. These findings suggest that the A and C regions of the skeletal DHPR II-III loop may have important roles in vivo.

Molecular cloning and functional expression of a skeletal muscle dihydropyridine receptor from Rana catesbeiana

In skeletal muscle the dihydropyridine receptor is the voltage sensor for excitation-contraction coupling and an L-type Ca2+ channel. We cloned a dihydropyridine receptor (named Fgalpha1S) from frog skeletal muscle, where excitation-contraction coupling has been studied most extensively. Fgalpha1S contains 5600 base pairs coding for 1688 amino acids. It is highly homologous with, and of the same length as, the C-truncated form predominant in rabbit muscle. The primary sequence has every feature needed to be an L-type Ca2+ channel and a skeletal-type voltage sensor. Currents expressed in tsA201 cells had rapid activation (5-10 ms half-time) and Ca2+-dependent inactivation. Although functional expression of the full Fgalpha1S was difficult, the chimera consisting of Fgalpha1S domain I in the rabbit cardiac Ca channel had high expression and a rapidly activating current. The slow native activation is therefore not determined solely by the alpha1 subunit sequence. Its Ca2+-dependent inactivation strengthens the notion that in rabbit skeletal muscle this capability is inhibited by a C-terminal stretch (Adams, B., and Tanabe, T. (1997) J. Gen. Physiol. 110, 379-389). This molecule constitutes a new tool for studies of excitation-contraction coupling, gating, modulation, and gene expression.

Localization in the II-III loop of the dihydropyridine receptor of a sequence critical for excitation-contraction coupling

Skeletal and cardiac muscles express distinct isoforms of the dihydropyridine receptor (DHPR), a type of voltage-gated Ca2+ channel that is important for excitation-contraction (EC) coupling. However, entry of Ca2+ through the channel is not required for skeletal muscle-type EC coupling. Previous work (Tanabe, T., Beam, K. G., Adams, B. A., Niidome, T., and Numa, S. (1990) Nature 346, 567-569) revealed that the loop between repeats II and III (II-III loop) is an important determinant of skeletal-type EC coupling. In the present study we have further dissected the regions of the II-III loop critical for skeletal-type EC coupling by expression of cDNA constructs in dysgenic myotubes. Because Ser687 of the skeletal II-III loop has been reported to be rapidly phosphorylated in vitro, we substituted this serine with alanine, the corresponding cardiac residue. This alanine-substituted skeletal DHPR retained the ability to mediate skeletal-type EC coupling. Weak skeletal-type EC coupling was produced by a chimeric DHPR, which was entirely cardiac except for a small amount of skeletal sequence (residues 725-742) in the II-III loop. Skeletal-type coupling was stronger when both residues 725-742 and adjacent residues were skeletal (e.g. a chimera containing skeletal residues 711-765). However, residues 725-742 appeared to be critical because skeletal-type coupling was not produced either by a chimera with skeletal residues 711-732 or by one with skeletal residues 734-765.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Differential effects of subunit interactions on protein kinase A- and C-mediated phosphorylation of L-type calcium channels

We have expressed the pore-forming alpha1S (skeletal muscle isoform) and alpha1C (cardiac/brain isoform) subunits, as well as the accessory beta2a (cardiac/brain isoform) and alpha2/delta subunits of the L-type, dihydropyridine-sensitive calcium (Ca) channels in Spodoptera frugiperda insect cells (Sf9 cells) by infection with recombinant baculoviruses in order to facilitate biochemical studies of these rare, heteromultimeric membrane proteins. Since the L-type channels are believed to be regulated by protein phosphorylation, this expression system allowed us to investigate which subunits could act as substrates for protein kinase A and C (PKA and PKC) and to determine the potential role of subunit interactions in phosphorylation of the channel proteins. Using purified protein kinases in vitro, the membrane-associated alpha1S, alpha1C, and beta2a subunits were demonstrated to be phosphorylated stoichiometrically by PKA. The extent of phosphorylation of these subunits by PKA was similar whether the subunits were expressed alone or in combination. In addition, the alpha1C and beta2a subunits were phosphorylated stoichiometrically by PKC when expressed individually. In contrast, the alpha1S subunit, when expressed alone, was a poor substrate for PKC, despite the fact that this subunit has been shown to be an excellent substrate for PKC in native skeletal muscle membranes. Interestingly, co-expression of alpha1S with the beta2a subunit restored the ability of the alpha1S subunit to serve as a substrate for PKC. These results strongly suggests that subunit interactions play an important and potentially differential role in channel regulation by PKC, whereas phosphorylation of the same subunit by PKA occurs independent of subunit interaction. Furthermore, our results provide biochemical evidence that, when co-expressed, the alpha1C, alpha1S, and beta2a subunits of L-type Ca2+ channels are excellent substrates for PKA and PKC and support the hypothesis that phosphorylation of each of these subunits may participate in channel regulation by these kinases.

Specific phosphorylation of a site in the full-length form of the alpha 1 subunit of the cardiac L-type calcium channel by adenosine 3',5'-cyclic monophosphate-dependent protein kinase

Voltage-gated L-type Ca2+ channels mediate Ca2+ entry into cells in response to membrane depolarization. Ca2+ entry through the cardiac Ca2+ channel determines the rate and force of contraction, and modulation of Ca2+ channel activity by beta-adrenergic agents acting through adenosine 3',5'-cyclic monophosphate-(cAMP)-dependent protein phosphorylation contributes to physiological regulation of cardiac function by the sympathetic nervous system. Immunoblotting experiments using site-directed anti-peptide antibodies against different peptide segments indicate that the alpha 1 subunit of the cardiac L-type Ca2+ channel exists in two size forms with apparent molecular masses of 240 and 210 kDa, which we call alpha 1(242) and alpha 1(210), Alpha 1(242) corresponds to the full-length cardiac alpha 1 subunit predicted from its cDNA sequence, while alpha 1(210) is truncated at its COOH terminus. Only alpha 1(242) is phosphorylated in vitro by cAMP-dependent protein kinase. Protein microsequencing and peptide mapping of wild-type and mutant fusion proteins show that this phosphorylation occurs at serine 1928 near the COOH terminus. Phosphorylation of this residue can be detected by phosphospecific antibodies raised against the corresponding phosphopeptide. Experiments with these antibodies show that alpha 1(242) is phosphorylated in intact cells expressing the cardiac alpha 1 subunit in response to increased intracellular levels of cAMP. These results identify serine 1928 on the alpha 1 subunit as a possible site of regulation by cAMP-dependent phosphorylation.

Cyclic AMP-dependent protein kinase phosphorylates residues in the C-terminal domain of the cardiac L-type calcium channel alpha1 subunit

The molecular basis of the regulation of cardiac L-type calcium channel activity by cAMP-dependent protein kinase (cA-PK) remains unclear. Direct cA-PK-dependent phosphorylation of the bovine ventricular alpha1 subunit in vitro has been demonstrated in microsomal membranes, detergent extracts and partially purified (+)-[3H]PN 200-110 receptor preparations. Two 32P-labeled phosphopeptides, derived from cyanogen bromide cleavage, of 4.7 and 9.5 kDa were immunoprecipitated specifically by site-directed antibodies against the rabbit cardiac alpha1 subunit amino acid sequences 1602-1616 and 1681-1694, respectively, consistent with phosphorylation at the cA-PK consensus sites at Ser(1627) and Ser(1700). No phosphopeptide products consistent with phosphorylation at three other C-terminal cA-PK consensus phosphorylation sites (Ser(1575), Ser(1848) and Ser(1928)) were identified using similar procedures suggesting that these sites are poor substrates for this kinase. Ser(1627) and Ser(1700) may represent sites of cA-PK phosphorylation involved in the physiological regulation of cardiac L-type calcium channel function.

A potential site of functional modulation by protein kinase A in the cardiac Ca2+ channel alpha 1C subunit

The well-characterized enhancement of the cardiac Ca2+ L-type current by protein kinase A (PKA) is not observed when the corresponding channel is expressed in Xenopus oocytes, possibly because it is fully phosphorylated in the basal state. However, the activity of the expressed channel is reduced by PKA inhibitors. Using this paradigm as an assay to search for PKA sites relevant to channel modulation, we have found that mutation of serine 1928 of the alpha 1C subunit to alanine abolishes the modulation of the expressed channel by PKA inhibitors. This effect was independent of the presence of the beta subunit. Phosphorylation of serine 1928 of alpha 1C may mediate the modulatory effect of PKA on the cardiac voltage-dependent ca2+ channel.

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.

Dihydropyridine receptor-ryanodine receptor interactions in skeletal muscle excitation-contraction coupling

Much recent progress has been made in our understanding of the mechanism of sarcoplasmic reticulum Ca2+ release in skeletal muscle. Vertebrate skeletal muscle excitation-contraction (E-C) coupling is thought to occur by a "mechanical coupling" mechanism involving protein-protein interactions that lead to activation of the sarcoplasmic reticulum (SR) ryanodine receptor (RyR)/Ca2+ release channel by the voltage-sensing transverse (T-) tubule dihydropyridine receptor (DHPR)/Ca2+ channel. In a subsequent step, the released Ca2+ amplify SR Ca2+ release by activating release channels that are not linked to the DHPR. Experiments with mutant muscle cells have indicated that skeletal muscle specific DHPR and RyR isoforms are required for skeletal muscle E-C coupling. A direct functional and structural interaction between a DHPR-derived peptide and the RyR has been described. The interaction between the DHPR and RyR may be stabilized by other proteins such as triadin (a SR junctional protein) and modulated by phosphorylation of the DHPR.

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.

Identification of calcium release-triggering and blocking regions of the II-III loop of the skeletal muscle dihydropyridine receptor

In an attempt to identify and characterize functional domains of the rabbit skeletal muscle dihydropyridine receptor alpha 1 subunit II-III loop, we synthesized several peptides corresponding to different regions of the loop: peptides A, B, C, C1, C2, D (cf. Fig. 1). Peptide A (Thr671-Leu690) activated [3H]ryanodine binding to, and induced Ca2+ release from, rabbit skeletal muscle triads, but none of the other peptides had such effects. Peptide A-induced Ca2+ release and activation of ryanodine binding were partially suppressed by an equimolar concentration of peptide C (Glu724-Pro760) but were not affected by the other peptides. These results suggest that the short stretch in the II-III loop, Thr671-Leu690, is responsible for triggering SR Ca2+ release, while the other region, Glu724-Pro760, functions as a blocker of the release trigger. A hypothesis is proposed to account for how these subdomains interact with the sarcoplasmic reticulum Ca2+ release channel protein during excitation-contraction coupling.

Phosphorylation of dihydropyridine receptor II-III loop peptide regulates skeletal muscle calcium release channel function. Evidence for an essential role of the beta-OH group of Ser687

In vertebrate skeletal muscle, excitation-contraction coupling may occur by a mechanical coupling mechanism involving protein-protein interactions between the dihydropyridine receptor (DHPR) of the transverse tubule membrane and the ryanodine receptor (RYR)/Ca2+ release channel of the sarcoplasmic reticulum membrane. We have previously shown that the cytoplasmic II-III loop peptides of the skeletal and cardiac muscle DHPR alpha 1 subunits (SDCL and CDCL, respectively) activate the skeletal muscle RYR. We now report that cyclic AMP-dependent protein kinase-mediated phosphorylation of Ser687 of SDCL yields a peptide that fails to activate the RYR, as determined in [3H]ryanodine binding and single channel measurements. The phosphorylated SDCL bound to the skeletal muscle but not cardiac muscle RYR, and the binding could be displaced by the unphosphorylated SDCL. A mutant SDCL with a Ser687-->Ala substitution failed to activate the RYR, but was still able to bind. Similarly, a Ser813-->Ala substitution in CDCL yielded a peptide that failed to activate the skeletal RYR. Use of three smaller overlapping peptides within the SDCL region identified an amino acid region from 666 to 726 including Ser687, which bound to and activated the skeletal muscle RYR. These results suggest that cyclic AMP-dependent protein kinase-mediated phosphorylation of the DHPR alpha 1 subunit may play a role in the functional interaction of the DHPR and RYR in skeletal muscle.

Sites of selective cAMP-dependent phosphorylation of the L-type calcium channel alpha 1 subunit from intact rabbit skeletal muscle myotubes

The principal (alpha 1) subunit of purified skeletal muscle dihydropyridine-sensitive (L-type) calcium channels is present in full-length (212 kDa) and COOH-terminal truncated (190 kDa) forms, which are both phosphorylated by cAMP-dependent protein kinase (cA-PK) in vitro. Immunoprecipitation of the calcium channel from rabbit muscle myotubes in primary cell culture followed by phosphorylation with cA-PK, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and two-dimensional phosphopeptide mapping revealed comparable phosphorylation of three COOH-terminal phosphopeptides found in the purified full-length alpha 1 subunit. Stimulation of muscle myotubes with a permeant cAMP analogue, 8-(4-chlorophenylthio) adenosine 3',5'-cyclic monophosphate, prior to immunoprecipitation of alpha 1 results in a 60-80% reduction of cA-PK catalyzed "back" phosphorylation of each of these sites in vitro in calcium channels purified from the cells, indicating that these sites are phosphorylated in vivo in response to increased intracellular cAMP. Serine 687, the most rapidly phosphorylated site in the truncated 190-kDa alpha 1 subunit, was observed as a minor phosphopeptide whose level of phosphorylation was not significantly affected by stimulation of endogenous cA-PK in the myotubes. The COOH-terminal sites, designated tryptic phosphopeptides 4, 5, and 6, were identified as serine 1757 (phosphopeptides 4 and 6) and 1854 (phosphopeptide 5) by a combination of protease cleavage, phosphorylation of synthetic peptides and fusion proteins, specific immunoprecipitation, and phosphopeptide mapping. Phosphorylation of serines 1757 and 1854 in the COOH-terminal region of the 212-kDa alpha 1 subunit in intact skeletal muscle cells may play a pivotal role in the regulation of calcium channel function by cA-PK.

Insect calcium channels. Molecular cloning of an alpha 1-subunit from housefly (Musca domestica) muscle

The complete amino acid sequence of an invertebrate calcium channel alpha 1-subunit from housefly (Musca domestica) larvae (designated Mdl alpha 1) has been deduced by cDNA cloning and sequence analysis. Mdl alpha 1 shares higher percent sequence identity with 1,4-dihydropyridine (DHP)-sensitive L-type than with DHP-insensitive calcium channels. As shown by whole mount in situ hybridization and immunostaining Mdl alpha 1 is predominantly expressed in the larval body wall musculature.

Phosphorylation of the L-type calcium channel beta subunit is involved in beta-adrenergic signal transduction in canine myocardium

Cyclic AMP-mediated phosphorylation of calcium channel subunits was studied in vitro and in vivo in preparations from dog heart. Calcium channels in native cardiac membranes were phosphorylated by cAMP-dependent protein kinase (PKA) solubilized with digitonin and subsequently immunoprecipitated using a polyclonal antibody generated against the deduced carboxy-terminal sequence of the cardiac beta subunit. A 62 kDa protein was identified as the major PKA-substrate in the immunoprecipitates. In the intact myocardium, this putative beta subunit was found to be phosphorylated in response to cAMP elevating agents. In contrast, no phosphorylation of a protein with an electrophoretic mobility similar to the alpha 1 subunit was detected, although 1,4-dihydropyridine receptor sites were recovered in the immunoprecipitates. Thus, we suggest that PKA-mediated phosphorylation of the beta subunit is the major mechanism for beta-adrenergic regulation of cardiac L-type calcium channel activity.

Selective dephosphorylation of the subunits of skeletal muscle calcium channels by purified phosphoprotein phosphatases

Multiple sites on the alpha 1 and beta subunits of purified skeletal muscle calcium channels are phosphorylated by cyclic AMP-dependent protein kinase, resulting in three different tryptic phosphopeptides derived from each subunit. Phosphoprotein phosphatases dephosphorylated these sites selectively. Phosphoprotein phosphatase 1 (PP1) and phosphoprotein phosphatase 2A (PP2A) dephosphorylated both alpha 1 and beta subunits at similar rates, whereas calcineurin dephosphorylated beta subunits preferentially. PP1 dephosphorylated phosphopeptides 1 and 2 of the alpha 1 subunit more rapidly than phosphopeptide 3. In contrast, PP2A dephosphorylated phosphopeptide 3 of the alpha 1 subunit preferentially. All three phosphoprotein phosphatases preferentially dephosphorylated phosphopeptide 1 of the beta subunit and dephosphorylated phosphopeptides 2 and 3 more slowly. Mn2+ increased the rate and extent of dephosphorylation of all sites by calcineurin so that > 80% dephosphorylation of both alpha 1 and beta subunits was obtained. The results demonstrate selective dephosphorylation of different phosphorylation sites on the alpha 1 and beta subunits of skeletal muscle calcium channels by the three principal serine/threonine phosphoprotein phosphatases.

Ion channels

Insulin secretion by the pancreatic Beta cell is dependent upon transmembrane ion fluxes gated by the ATP-regulated potassium channel and the voltage regulated, L-type calcium channel. This work group examined major recent advances in the structure and modulation of ion channels and how those advances may pertain to the physiology of insulin secretion and the pharmacological treatment of Type 2 (non-insulin-dependent) diabetes mellitus. Structural studies have revealed that voltage gated ion channels are related, complex, and comprised of multiple components: sodium channels consist of three distinct subunits. L-type calcium channels, crucial to the insulin secretory response are structurally related to the sodium channel but contain additional subunits. Potassium channels are less closely related and appear to function as homotetramers. Modulation of ion channel activity is similarly complex: site specific phosphorylation by multiple protein kinases under the control of several intracellular second messenger systems may increase or decrease conductance. Subunit composition and relatively stable changes in the modal state of ion channels also appear to be critical to ion channel gating properties. Functional studies of the Beta-cell ATP-regulated potassium channel suggest two distinct nucleotide binding sites which link this channel to the metabolic state of the Beta cell. The multiple paths of ion channel modulation provide multiple targets for therapeutic intervention. Where detailed characterisation of ion channel structure has been achieved, those targets are being used for specific drug design. Such complete characterisation has not yet been achieved for Beta-cell ion channels and this presents a major goal for diabetes research.

Cyclic AMP-dependent phosphorylation and regulation of the cardiac dihydropyridine-sensitive Ca channel

A polyclonal antibody, CR2, prepared using the C-terminal peptide of the alpha 1 subunit of the rabbit cardiac DHP-sensitive Ca channel, specifically immunoprecipitated the [3H]PN200-110-labeled Ca channel solubilized from cardiac microsomes. The antibody recognized 250 and 200-kDa cardiac microsomal proteins as determined by immunoblotting, and cAMP-dependent protein kinase phosphorylated the 250-kDa, but not the 200-kDa protein in vitro. CHO cells, transfected with the cardiac alpha 1 subunit cDNA carried by an expression vector, synthesized a 250-kDa protein which was recognized by CR2. Adding db-cAMP or forskolin to the transformed CHO cells induced phosphorylation of the 250-kDa protein and stimulated the DHP-sensitive Ba current under patch-clamp conditions. These results suggested that the cardiac DHP-sensitive Ca channel was regulated by cAMP-dependent phosphorylation of the alpha 1 subunit.

Characterization of the myotonic dystrophy region predicts multiple protein isoform-encoding mRNAs

The mutation underlying myotonic dystrophy (DM) has been identified as an expansion of a polymorphic CTG-repeat in a gene encoding protein kinase activity. Brain and heart transcripts of the DM-kinase (DMR-B15) gene are subject to alternative RNA splicing in both human and mouse. The unstable [CTG]5-30 motif is found uniquely in humans, although the flanking nucleotides are also present in mouse. Characterization of the DM region of both species reveals another active gene (DMR-N9) in close proximity to the kinase gene. DMR-N9 transcripts, mainly expressed in brain and testis, possess a single, large open reading frame, but the function of its protein product is unknown. Clinical manifestation of DM may be caused by the expanded CTG-repeat compromising the (alternative) expression of DM-kinase or DMR-N9 proteins.

A rapid procedure for the purification of cardiac 1,4-dihydropyridine receptors from porcine heart

Highly purified porcine cardiac sarcolemma was used as a source for purification of mammalian cardiac 1,4-dihydropyridine receptors associated with the voltage-dependent Ca2+ channel. The cardiac digitonin-solubilized receptor prelabeled with (+)-[3H]PN 200-110 was enriched at least 236-fold using an improved, rapid three-step purification protocol which could be completed within 12 h. The purity of the preparation was at least 22%, the yield of the receptors 24%. Photoaffinity labeling experiments with (-)-[3H]azidopine allowed the identification of the cardiac alpha 1 subunit. In contrast to the purified rabbit or guinea-pig skeletal muscle Ca2+ channel complex, none of the purified polypeptides underwent rapid and substantial phosphorylation by the catalytic subunit of the cyclic AMP-dependent protein kinase in vitro.

The mechanical hypothesis of excitation-contraction (EC) coupling in skeletal muscle

The mechanism of transmission in skeletal muscle EC coupling is still an open question. There is some indirect evidence in favour of the mechanical coupling hypothesis, deriving mostly from consideration of the structure of the Ca2+ release channel protein. A new functional approach is proposed, that consists in comparing the properties of the complete system--EC coupling in a skeletal muscle fibre--with those of the EC coupling molecules in bilayers. In this approach, those properties of the whole system that are not traceable to its constitutive molecules, are ascribed to the physiological interaction, and are expected to yield new information on the nature of this interaction.