The mouse sino-atrial node expresses both the type 2 and type 3 Ca(2+) release channels/ryanodine receptors

Ca(2+) released from intracellular Ca(2+) stores is shown to be involved in pacemaker activity in the sino-atrial (SA)-node. However, little is known about the molecular identity of the Ca(2+) release channel/ryanodine receptor (RYR) involved in pacemaker activity. We examined the mRNA distribution of three different RYR isoforms (RYR1, RYR2, and RYR3) in the mouse SA-node. RNase protection assay and in situ hybridization revealed that RYR2 mRNA expresses widely in the heart including the SA-node, while RYR3 mRNA expression is limited to the SA-node and to the right atrium. Thus, not only RYR2 but also RYR3 may participate in pacemaker activity.

Three-dimensional localization of divergent region 3 of the ryanodine receptor to the clamp-shaped structures adjacent to the FKBP binding sites

Of the three divergent regions of ryanodine receptors (RyRs), divergent region 3 (DR3) is the best studied and is believed to be involved in excitation-contraction coupling as well as in channel regulation by Ca(2+) and Mg(2+). To gain insight into the structural basis of DR3 function, we have determined the location of DR3 in the three-dimensional structure of RyR2. We inserted green fluorescent protein (GFP) into the middle of the DR3 region after Thr-1874 in the sequence. HEK293 cells expressing this GFP-RyR2 fusion protein, RyR2(T1874-GFP,) were readily detected by their green fluorescence, indicating proper folding of the inserted GFP. RyR2(T1874-GFP) was further characterized functionally by assays of Ca(2+) release and [(3)H]ryanodine binding. These analyses revealed that RyR2(T1874-GFP) functions as a caffeine- and ryanodine-sensitive Ca(2+) release channel and displays Ca(2+) dependence and [(3)H]ryanodine binding properties similar to those of the wild type RyR2. RyR2(T1874-GFP) was purified from cell lysates in a single step by affinity chromatography using GST-FKBP12.6 as the affinity ligand. The three-dimensional structure of the purified RyR2(T1874-GFP) was then reconstructed using cryoelectron microscopy and single particle image analysis. Comparison of the three-dimensional reconstructions of wild type RyR2 and RyR2(T1874-GFP) revealed the location of the inserted GFP, and hence the DR3 region, in one of the characteristic domains of RyR, domain 9, in the clamp-shaped structure adjacent to the FKBP12 and FKBP12.6 binding sites. COOH-terminal truncation analysis demonstrated that a region between 1815 and 1855 near DR3 is essential for GST-FKBP12.6 binding. These results provide a structural basis for the role of the DR3 region in excitation-contraction coupling and in channel regulation.

Calmodulin regulation of excitation-contraction coupling in cardiac myocytes

Calmodulin (CaM) as a ubiquitous Ca2+ sensor interacts with multiple key molecules involved in excitation-contraction (EC) coupling. In the present study, we report that adenoviral expression of a mutant CaM lacking all of its four Ca2+-binding sites, CaM(1-4), at a level 6.5-fold over endogenous CaM markedly increases the amplitude and abbreviates the decay time of Ca2+ transients and contraction in cultured rat ventricular myocytes. To determine the underlying mechanisms, we examined the properties of L-type Ca2+ channels, Ca2+/CaM-dependent protein kinase II (CaMKII), and phospholamban (PLB) in the sarcoplasmic reticulum (SR). We found that CaM(1-4) expression markedly augmented L-type Ca2+ current amplitude and slowed its inactivation. Surprisingly, overexpression of CaM(1-4) increased CaMKII activity and phosphorylation of PLB-Thr-17. Moreover, CaM(1-4) elevated diastolic Ca2+ and caffeine-labile Ca2+ content of the SR. Inhibition of CaMKII by KN-93 or a myristoylated autocamtide-2 related inhibitory peptide prevented the aforementioned PLB phosphorylation and reversed the positive inotropic and relaxant effects, indicating that CaMKII is essential to CaM(1-4) actions. These results demonstrate that CaM modulates Ca2+ influx, SR Ca2+ release, and Ca2+ recycling during cardiac EC coupling. A novel finding of this study is that expression of a Ca2+-insensitive CaM mutant can lead to activation of CaMKII in cardiac myocytes.

Smooth muscle tissues express a major dominant negative splice variant of the type 3 Ca2+ release channel (ryanodine receptor)

It is well known that the type 3 Ca(2+) release channel (ryanodine receptor, RyR3) exhibits strikingly different pharmacological and functional properties depending on the tissues in which it resides. To investigate the molecular basis for this tissue-dependent heterogeneity, we examined the primary structure of RyR3 from various tissues by reverse transcription polymerase chain reaction and DNA sequence analysis. As many as seven alternatively spliced variants of RyR3 were detected. Ribonuclease protection assays revealed that one of these splice variants, RyR3 (AS-8a), which lacks a 29-amino acid fragment (His(4406)-Lys(4434)) encompassing a predicted transmembrane helix, was highly expressed in smooth muscle tissues, but not in skeletal muscle, the heart, or the brain. Although the RyR3 (AS-8a) splice variant did not form a functional Ca(2+) release channel when expressed alone in HEK293 cells, it was able to form functional heteromeric channels with reduced caffeine sensitivity when co-expressed with the wild type RyR3. Interestingly, this RyR3 splice variant was also able to form heteromeric channels with and suppress the activity of the type 2 ryanodine receptor (RyR2). Tissue-specific expression of RyR3 splice variants is therefore likely to account for some of the pharmacological and functional heterogeneities of RyR3. These observations also reveal a novel mechanism by which a splice variant of one RyR isoform (RyR3) can suppress the activity of another RyR isoform (RyR2) via a dominant negative effect.

Localization of the 12.6-kDa FK506-binding protein (FKBP12.6) binding site to the NH2-terminal domain of the cardiac Ca2+ release channel (ryanodine receptor)

The 12.6-kDa FK506-binding protein (FKBP12.6) interacts with the cardiac ryanodine receptor (RyR2) and modulates its channel function. However, the molecular basis of FKBP12.6-RyR2 interaction is poorly understood. To investigate the significance of the isoleucine-proline (residues 2427-2428) dipeptide epitope, which is thought to form an essential part of the FKBP12.6 binding site in RyR2, we generated single and double mutants, P2428Q, I2427E/P2428A, and P2428A/L2429E, expressed them in HEK293 cells, and assessed their ability to bind GST-FKBP12.6. None of these mutations abolished GST-FKBP12.6 binding, indicating that this isoleucine-proline motif is unlikely to form the core of the FKBP12.6 binding site in RyR2. To systematically define the molecular determinants of FKBP12.6 binding, we constructed a series of internal and NH(2)- and COOH-terminal deletion mutants of RyR2 and examined the effect of these deletions on GST-FKBP12.6 binding. These deletion analyses revealed that the first 305 NH(2)-terminal residues and COOH-terminal residues 1937-4967 are not essential for GST-FKBP12.6 binding, whereas multiple sequences within a large region between residues 305 and 1937 are required for GST-FKBP12.6 interaction. Furthermore, an NH(2)-terminal fragment containing the first 1937 residues is sufficient for GST-FKBP12.6 binding. Co-expression of overlapping NH(2) and COOH-terminal fragments covering the entire sequence of RyR2 produced functional channels but did not restore GST-FKBP12.6 binding. These data suggest that FKBP12.6 binding is likely to be conformationdependent. Binding of FKBP12.6 to the NH(2)-terminal domain may play a role in stabilizing the conformation of this region.

Three-dimensional reconstruction of the recombinant type 2 ryanodine receptor and localization of its divergent region 1

Isoform 2 of the ryanodine receptor (RyR2) is the major calcium release channel in cardiac muscle. In the present study, two kinds of RyR2 cDNA were constructed, one encoding the wild type mouse RyR2 (RyR2(wt)) and the other encoding modified RyR2, into which was inserted a cDNA encoding green fluorescent protein (GFP). GFP was inserted into the divergent region 1 (DR1) of RyR2, after the Asp-4365 (RyR2(D4365-GFP)). HEK293 cells expressing both RyR2(wt) and RyR2(D4365-GFP) cDNAs showed caffeine- and ryanodine-sensitive calcium release, demonstrating that both wild type and modified RyR2s form functional calcium release channels. Cells expressing the fusion protein, RyR2(D4365-GFP), were readily identified by their fluorescence due to the presence of GFP, indicating that the inserted GFP folded properly. Both expressed RyR2s were purified from cell lysates in a single step by affinity chromatography using a GST-FKBP12.6 as the affinity ligand. Cryoelectron microscopy of purified RyR2s showed structurally intact receptors, and three-dimensional reconstructions were obtained by single particle image processing. The three-dimensional reconstruction of RyR2(wt) appeared very similar to that of the native RyR2 purified from dog heart. The location of the inserted GFP, and consequently of DR1, was mapped on the three-dimensional structure of RyR2 to one of the subunit's characteristic domains, domain 3, also known as the "handle" domain. This study describes the first internal fusion of a protein into a ryanodine receptor, and it demonstrates the potential of this technology for localizing functional and structural domains on the three-dimensional structure of RyR.

Isoform-dependent formation of heteromeric Ca2+ release channels (ryanodine receptors)

Three ryanodine receptor (RyR) isoforms, RyR1, RyR2, and RyR3, are expressed in mammalian tissues. It is unclear whether RyR isoforms are capable of forming heteromeric channels. To investigate their ability to form heteromeric channels, we co-expressed different RyR isoforms in HEK293 cells and examined their interactions biochemically and functionally. Immunoprecipitation studies revealed that RyR2 is able to interact physically with RyR3 and RyR1 in HEK293 cells and that RyR1 does not interact with RyR3. Co-expression of a ryanodine binding deficient mutant of RyR2, RyR2 (I4827T), with RyR3 (wt) restored [(3)H]ryanodine binding to the mutant. Interactions between RyR isoforms were further assessed by complementation analysis using mutants RyR2 (I4827T), RyR2 (E3987A), RyR3 (I4732T), RyR3 (E3885A), and RyR1 (E4032A), all of which are deficient in caffeine response. Caffeine-induced Ca(2+) release was restored in HEK293 cells co-transfected with mutants RyR2 (I4827T) and RyR3 (E3885A), RyR2 (E3987A) and RyR3 (I4732T), or RyR2 (I4827T) and RyR1 (E4032A), but not with RyR1 (E4032A) and RyR3 (I4732T), indicating that mutants of RyR2 and RyR3, or RyR2 and RyR1, but not RyR1 and RyR3, are able to complement each other. Co-expression of RyR3 (wt) and a pore mutant of RyR2, RyR2 (G4824A), produced regulatable single channels with intermediate unitary conductances. These observations demonstrate that RyR2 is capable of forming functional heteromeric channels with RyR3 and RyR1, whereas RyR1 is incapable of forming heteromeric channels with RyR3.

Enhanced basal activity of a cardiac Ca2+ release channel (ryanodine receptor) mutant associated with ventricular tachycardia and sudden death

Mutations in the human cardiac Ca2+ release channel (ryanodine receptor, RyR2) gene have recently been shown to cause effort-induced ventricular arrhythmias. However, the consequences of these disease-causing mutations in RyR2 channel function are unknown. In the present study, we characterized the properties of mutation R4496C of mouse RyR2, which is equivalent to a disease-causing human RyR2 mutation R4497C, by heterologous expression of the mutant in HEK293 cells. [3H]ryanodine binding studies revealed that the R4496C mutation resulted in an increase in RyR2 channel activity in particular at low Ca2+ concentrations. This increased basal channel activity remained sensitive to modulation by caffeine, ATP, Mg2+, and ruthenium red. In addition, the R4496C mutation enhanced the sensitivity of RyR2 to activation by Ca2+ and by caffeine. Single-channel analysis showed that single R4496C mutant channels exhibited considerable channel openings at low Ca2+ concentrations. HEK293 cells transfected with mutant R4496C displayed spontaneous Ca2+ oscillations more frequently than cells transfected with wild-type RyR2. Substitution of a negatively charged glutamate for the positively charged R4496 (R4496E) further enhanced the basal channel activity, whereas replacement of R4496 by a positively charged lysine (R4496K) had no significant effect on the basal activity. These observations indicate that the charge and polarity at residue 4496 plays an essential role in RyR2 channel gating. Enhanced basal activity of RyR2 may underlie an arrhythmogenic mechanism for effort-induced ventricular tachycardia.

Role of the proposed pore-forming segment of the Ca2+ release channel (ryanodine receptor) in ryanodine interaction

In earlier studies we showed that point mutations introduced into the proposed pore-forming segment, GVRAGGGIGD (amino acids 4820-4829), of the mouse cardiac ryanodine receptor reduced or abolished high affinity [3H]ryanodine binding. Here we investigate the effects of these mutations on the affinity and dissociation properties of [3H]ryanodine binding and on ryanodine modification of the ryanodine receptor channel at the single channel and whole cell levels. Scatchard analysis and dissociation studies reveal that mutation G4824A decreases the equilibrium dissociation constant (K(d)) and the dissociation rate constant (k(off)), whereas mutations G4828A and D4829A increase the K(d) and k(off) values. The effect of ryanodine on single G4828A and D4829A mutant channels is reversible on the time scale of single channel experiments, in contrast to the irreversible effect of ryanodine on single wild-type channels. Ryanodine alone is able to induce a large and sustained Ca2+ release in HEK293 cells transfected with the R4822A or G4825A mutant cDNA at the resting cytoplasmic Ca2+ but causes little or no Ca2+ release in cells transfected with the wild-type cDNA. Mutation G4826C diminishes the functional effect of ryanodine on Ca2+ release but spares caffeine-induced Ca2+ release in HEK293 cells. Co-expression of the wild-type and G4826C mutant proteins produces single channels that interact with ryanodine reversibly and display altered conductance and ryanodine response. These results are consistent with the view that the proposed pore-forming segment is a critical determinant of ryanodine interaction. A putative model of ryanodine-ryanodine receptor interaction is proposed.

Ca2+ activation of RyR1 is not necessary for the initiation of skeletal-type excitation-contraction coupling

Although an elevation in myoplasmic Ca2+ can activate the skeletal muscle ryanodine receptor (RyR1), the function of this Ca2+ activation is unclear because extracellular Ca2+ influx is unnecessary for skeletal-type EC coupling. To determine whether Ca2+ activation of RyR1 is necessary for the initiation of skeletal-type EC coupling, we examined the behavior of RyR1 with glutamate 4032 mutated to alanine (E4032A-RyR1) because this mutation had been shown to dramatically reduce activation by Ca2+. Proc. Natl. Acad. Sci. USA. 98:2865-2870). Analysis after reconstitution into planar lipid bilayers revealed that E4032A-RyR1 was negligibly activated by 100 microM Ca2+ (P(o) too low to be measured). Even in the presence of both 2 mM caffeine and 2 mM ATP, P(o) remained low for E4032A-RyR1 (ranging from <0.0001 in 100 microM free Ca2+ to 0.005 in 2 mM free Ca2+). Thus, the E4032A mutation caused a nearly complete suppression of activation of RyR1 by Ca2+. Depolarization of E4032A-RyR1-expressing myotubes elicited L-type Ca2+ currents of approximately normal size and myoplasmic Ca2+ transients that were skeletal-type, but about fivefold smaller than those for wild-type RyR1. The reduced amplitude of the Ca2+ transient is consistent either with the possibility that Ca2+ activation amplifies Ca2+ release during EC coupling, or that the E4032A mutation generally inhibits activation of RyR1. In either case, Ca2+ activation of RyR1 does not appear to be necessary for the initiation of Ca2+ release during EC coupling in skeletal muscle.