CPVT-associated calmodulin variants N53I and A102V dysregulate Ca2+ signalling via different mechanisms

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited condition that can cause fatal cardiac arrhythmia. Human mutations in the Ca2+ sensor calmodulin (CaM) have been associated with CPVT susceptibility, suggesting that CaM dysfunction is a key driver of the disease. However, the detailed molecular mechanism remains unclear. Focusing on the interaction with the cardiac ryanodine receptor (RyR2), we determined the effect of CPVT-associated variants N53I and A102V on the structural characteristics of CaM and on Ca2+ fluxes in live cells. We provide novel data showing that interaction of both Ca2+/CaM-N53I and Ca2+/CaM-A102V with the RyR2 binding domain is decreased. Ca2+/CaM-RyR23583-3603 high-resolution crystal structures highlight subtle conformational changes for the N53I variant, with A102V being similar to wild type (WT). We show that co-expression of CaM-N53I or CaM-A102V with RyR2 in HEK293 cells significantly increased the duration of Ca2+ events; CaM-A102V exhibited a lower frequency of Ca2+ oscillations. In addition, we show that CaMKIIδ (also known as CAMK2D) phosphorylation activity is increased for A102V, compared to CaM-WT. This paper provides novel insight into the molecular mechanisms of CPVT-associated CaM variants and will facilitate the development of strategies for future therapies.

Classification and correlation of RYR2 missense variants in individuals with catecholaminergic polymorphic ventricular tachycardia reveals phenotypic relationships

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is predominantly caused by heterozygous missense variants in the cardiac ryanodine receptor, RYR2. However, many RYR2 missense variants are classified as variants of uncertain significance (VUS). We systematically re-evaluated all RYR2 variants in healthy individuals and those with CPVT or arrhythmia using the 2015 American College of Medical Genomics guidelines. RYR2 variants were identified by the NW Genomic Laboratory Hub, from the published literature and databases of sequence variants. Each variant was assessed based on minor allele frequencies, in silico prediction tools and appraisal of functional studies and classified according to the ACMG-AMP guidelines. Phenotype data was collated where available. Of the 326 identified RYR2 missense variants, 55 (16.9%), previously disease-associated variants were reclassified as benign. Application of the gnomAD database of >140,000 controls allowed reclassification of 11 variants more than the ExAC database. CPVT-associated RYR2 variants clustered predominantly between amino acid positions 3949-4332 and 4867-4967 as well as the RyR and IP3R homology-associated and ion transport domains (p < 0.005). CPVT-associated RYR2 variants occurred at more conserved amino acid positions compared with controls, and variants associated with sudden death had higher conservation scores (p < 0.005). There were five potentially pathogenic RYR2 variants associated with sudden death during sleep which were located almost exclusively in the C-terminus of the protein. In conclusion, control sequence databases facilitate reclassification of RYR2 variants but the majority remain as VUS. Notably, pathogenic variants in RYR2 are associated with death in sleep.

Calsequestrin interacts directly with the cardiac ryanodine receptor luminal domain

Cardiac muscle contraction requires sarcoplasmic reticulum (SR) Ca²⁺ release mediated by the quaternary complex comprising the ryanodine receptor 2 (RyR2), calsequestrin 2 (CSQ2), junctin (encoded by ASPH) and triadin. Here, we demonstrate that a direct interaction exists between RyR2 and CSQ2. Topologically, CSQ2 binding occurs at the first luminal loop of RyR2. Co-expression of RyR2 and CSQ2 in a human cell line devoid of the other quaternary complex proteins results in altered Ca²⁺-release dynamics compared to cells expressing RyR2 only. These findings provide a new perspective for understanding the SR luminal Ca²⁺ sensor and its involvement in cardiac physiology and disease.

Effect of flecainide derivatives on sarcoplasmic reticulum calcium release suggests a lack of direct action on the cardiac ryanodine receptor

BACKGROUND AND PURPOSE

Flecainide is a use-dependent blocker of cardiac Na(+) channels. Mechanistic analysis of this block showed that the cationic form of flecainide enters the cytosolic vestibule of the open Na(+) channel. Flecainide is also effective in the treatment of catecholaminergic polymorphic ventricular tachycardia but, in this condition, its mechanism of action is contentious. We investigated how flecainide derivatives influence Ca(2) (+) -release from the sarcoplasmic reticulum through the ryanodine receptor channel (RyR2) and whether this correlates with their effectiveness as blockers of Na(+) and/or RyR2 channels.

EXPERIMENTAL APPROACH

We compared the ability of fully charged (QX-FL) and neutral (NU-FL) derivatives of flecainide to block individual recombinant human RyR2 channels incorporated into planar phospholipid bilayers, and their effects on the properties of Ca(2) (+) sparks in intact adult rat cardiac myocytes.

KEY RESULTS

Both QX-FL and NU-FL were partial blockers of the non-physiological cytosolic to luminal flux of cations through RyR2 channels but were significantly less effective than flecainide. None of the compounds influenced the physiologically relevant luminal to cytosol cation flux through RyR2 channels. Intracellular flecainide or QX-FL, but not NU-FL, reduced Ca(2) (+) spark frequency.

CONCLUSIONS AND IMPLICATIONS

Given its inability to block physiologically relevant cation flux through RyR2 channels, and its lack of efficacy in blocking the cytosolic-to-luminal current, the effect of QX-FL on Ca(2) (+) sparks is likely, by analogy with flecainide, to result from Na(+) channel block. Our data reveal important differences in the interaction of flecainide with sites in the cytosolic vestibules of Na(+) and RyR2 channels.

The mechanism of flecainide action in CPVT does not involve a direct effect on RyR2

RATIONALE

Flecainide, a class 1c antiarrhythmic, has emerged as an effective therapy in preventing arrhythmias in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT) refractory to β-adrenergic receptor blockade. It has been proposed that the clinical efficacy of flecainide in CPVT is because of the combined actions of direct blockade of ryanodine receptors (RyR2) and Na(+) channel inhibition. However, there is presently no direct evidence to support the notion that flecainide blocks RyR2 Ca(2+) flux in the physiologically relevant (luminal-to-cytoplasmic) direction. The mechanism of flecainide action remains controversial.

OBJECTIVE

To examine, in detail, the effect of flecainide on the human RyR2 channel and to establish whether the direct blockade of physiologically relevant RyR2 ion flow by the drug contributes to its therapeutic efficacy in the clinical management of CPVT.

METHODS AND RESULTS

Using single-channel analysis, we show that, even at supraphysiological concentrations, flecainide did not inhibit the physiologically relevant, luminal-to-cytosolic flux of cations through the channel. Moreover, flecainide did not alter RyR2 channel gating and had negligible effect on the mechanisms responsible for the sarcoplasmic reticulum charge-compensating counter current. Using permeabilized cardiac myocytes to eliminate any contribution of plasmalemmal Na(+) channels to the observed actions of the drug at the cellular level, flecainide did not inhibit RyR2-dependent sarcoplasmic reticulum Ca(2+) release.

CONCLUSIONS

The principal action of flecainide in CPVT is not via a direct interaction with RyR2. Our data support a model of flecainide action in which Na(+)-dependent modulation of intracellular Ca(2+) handling attenuates RyR2 dysfunction in CPVT.

Dantrolene rescues aberrant N-terminus intersubunit interactions in mutant pro-arrhythmic cardiac ryanodine receptors

AIMS

The ryanodine receptor (RyR2) is an intracellular Ca(2+) release channel essential for cardiac excitation-contraction coupling. Abnormal RyR2 channel function results in the generation of arrhythmias and sudden cardiac death. The present study was undertaken to investigate the mechanistic basis of RyR2 dysfunction in inherited arrhythmogenic cardiac disease.

METHODS AND RESULTS

We present several lines of complementary evidence, indicating that the arrhythmia-associated L433P mutation disrupts RyR2 N-terminus self-association. A combination of yeast two-hybrid, co-immunoprecipitation, and chemical cross-linking assays collectively demonstrate that a RyR2 N-terminal fragment carrying the L433P mutation displays substantially reduced self-interaction compared with wild type. Moreover, sucrose density gradient centrifugation reveals that the L433P mutation impairs tetramerization of the full-length channel. [(3)H]Ryanodine-binding assays demonstrate that disrupted N-terminal intersubunit interactions within RyR2(L433P) confer an altered sensitivity to Ca(2+) activation. Calcium imaging of RyR2(L433P)-expressing cells reveals substantially prolonged Ca(2+) transients and reduced Ca(2+) store content indicating defective channel closure. Importantly, dantrolene treatment reverses the L433P mutation-induced impairment and restores channel function.

CONCLUSION

The N-terminus domain constitutes an important structural determinant for the functional oligomerization of RyR2. Our findings are consistent with defective N-terminus self-association as a molecular mechanism underlying RyR2 channel deregulation in inherited arrhythmogenic cardiac disease. Significantly, the therapeutic action of dantrolene may occur via the restoration of normal RyR2 N-terminal intersubunit interactions.

Insights into the gating mechanism of the ryanodine-modified human cardiac Ca2+-release channel (ryanodine receptor 2)

Ryanodine receptors (RyRs) are intracellular membrane channels playing key roles in many Ca(2+) signaling pathways and, as such, are emerging novel therapeutic and insecticidal targets. RyRs are so named because they bind the plant alkaloid ryanodine with high affinity and although it is established that ryanodine produces profound changes in all aspects of function, our understanding of the mechanisms underlying altered gating is minimal. We address this issue using detailed single-channel gating analysis, mathematical modeling, and energetic evaluation of state transitions establishing that, with ryanodine bound, the RyR pore adopts an extremely stable open conformation. We demonstrate that stability of this state is influenced by interaction of divalent cations with both activating and inhibitory cytosolic sites and, in the absence of activating Ca(2+), trans-membrane voltage. Comparison of the conformational stability of ryanodine- and Imperatoxin A-modified channels identifies significant differences in the mechanisms of action of these qualitatively similar ligands.

N-terminus oligomerization regulates the function of cardiac ryanodine receptors

The ryanodine receptor (RyR) is an ion channel composed of four identical subunits mediating calcium efflux from the endo/sarcoplasmic reticulum of excitable and non-excitable cells. We present several lines of evidence indicating that the RyR2 N-terminus is capable of self-association. A combination of yeast two-hybrid screens, co-immunoprecipitation analysis, chemical crosslinking and gel filtration assays collectively demonstrate that a RyR2 N-terminal fragment possesses the intrinsic ability to oligomerize, enabling apparent tetramer formation. Interestingly, N-terminus tetramerization mediated by endogenous disulfide bond formation occurs in native RyR2, but notably not in RyR1. Disruption of N-terminal inter-subunit interactions within RyR2 results in dysregulation of channel activation at diastolic Ca(2+) concentrations from ryanodine binding and single channel measurements. Our findings suggest that the N-terminus interactions mediating tetramer assembly are involved in RyR channel closure, identifying a crucial role for this structural association in the dynamic regulation of intracellular Ca(2+) release.

Investigations of the contribution of a putative glycine hinge to ryanodine receptor channel gating

Ryanodine receptor channels (RyR) are key components of striated muscle excitation-contraction coupling, and alterations in their function underlie both inherited and acquired disease. A full understanding of the disease process will require a detailed knowledge of the mechanisms and structures involved in RyR function. Unfortunately, high-resolution structural data, such as exist for K⁺-selective channels, are not available for RyR. In the absence of these data, we have used modeling to identify similarities in the structural elements of K⁺ channel pore-forming regions and postulated equivalent regions of RyR. This has identified a sequence of residues in the cytosolic cavity-lining transmembrane helix of RyR (G⁴⁸⁶⁴LIIDA⁴⁸⁶⁹ in RyR2) analogous to the glycine hinge motif present in many K⁺ channels. Gating in these K⁺ channels can be disrupted by substitution of residues for the hinge glycine. We investigated the involvement of glycine 4864 in RyR2 gating by monitoring properties of recombinant human RyR2 channels in which this glycine is replaced by residues that alter gating in K⁺ channels. Our data demonstrate that introducing alanine at position 4864 produces no significant change in RyR2 function. In contrast, function is altered when glycine 4864 is replaced by either valine or proline, the former preventing channel opening and the latter modifying both ion translocation and gating. Our studies reveal novel information on the structural basis of RyR gating, identifying both similarities with, and differences from, K⁺ channels. Glycine 4864 is not absolutely required for channel gating, but some flexibility at this point in the cavity-lining transmembrane helix is necessary for normal RyR function.

A mechanistic description of gating of the human cardiac ryanodine receptor in a regulated minimal environment

Cardiac muscle contraction, triggered by the action potential, is mediated by the release of Ca²⁺ from the sarcoplasmic reticulum through ryanodine receptor (RyR)2 channels. In situ, RyR2 gating is modulated by numerous physiological and pharmacological agents, and altered RyR2 function underlies the occurrence of arrhythmias in both inherited and acquired diseases. To understand fully the mechanisms underpinning the regulation of RyR2 in the normal heart and how these systems are altered in pathological conditions, we must first gain a detailed knowledge of the fundamental processes of RyR2 gating. In this investigation, we provide key novel mechanistic insights into the physical reality of RyR2 gating revealed by new experimental and analytical approaches. We have examined in detail the single-channel gating kinetics of the purified human RyR2 when activated by cytosolic Ca²⁺ in a stringently regulated environment where the modulatory influence of factors external to the channel were minimized. The resulting gating schemes are based on an accurate description of single-channel kinetics using hidden Markov model analysis and reveal several novel aspects of RyR2 gating behavior: (a) constitutive gating is observed as unliganded opening events; (b) binding of Ca²⁺ to the channel stabilizes it in different open states; (c) RyR2 exists in two preopening closed conformations in equilibrium, one of which binds Ca²⁺ more readily than the other; (d) the gating of RyR2 when bound to Ca²⁺ can be described by a kinetic scheme incorporating bursts; and (e) analysis of flicker closing events within bursts reveals gating activity that is not influenced by ligand binding. The gating schemes generated in this investigation provide a framework for future studies in which the mechanisms of action of key physiological regulatory factors, disease-linked mutations, and potential therapeutic compounds can be described precisely.

Mapping a quantitative trait locus for the concentration of beta-lactoglobulin in milk, and the effect of beta-lactoglobulin genetic variants on the composition of milk from Holstein-Friesian x Jersey crossbred cows

AIM

To identify quantitative trait loci (QTL) affecting the concentration of beta-lactoglobulin in milk, and to evaluate the effect of beta-lactoglobulin genetic variants on the concentration of fat, protein and casein in bovine milk.

METHODS

A herd of 850 F2 Holstein-Friesian x Jersey crossbred cows was produced through mating six Holstein-Friesian x Jersey F1 bulls of high genetic merit with F1 cows from the national herd. A total of 1,610 herd-test records from 556 second-parity crossbreds were analysed. The concentration of fat, protein and casein in milk was measured at peak, mid- and late lactation, during the production seasons of 2003-2004 and 2004-2005. Liveweight was measured daily. DNA from the F2 animals, their F1 dams and sires, and selected grandsires was genotyped across the genome, initially with 285 microsatellite markers, and subsequently with 6,634 single nucleotide polymorphisms (SNP).

RESULTS

A highly significant QTL for the concentration of beta-lactoglobulin in milk was identified, which coincided with the position of the beta-lactoglobulin gene on bovine Chromosome 11. No other consistently significant QTL for the concentration of beta-lactoglobulin in milk were detected. Cows with the BB beta-lactoglobulin genotype produced milk with a 30% lower concentration of beta-lactoglobulin than cows with the AA genotype. The beta-lactoglobulin polymorphism also explained variation in the proportion of casein in total protein. In addition, the percentage of fat was higher for BB than AA animals, whereas the percentage of total protein, mean daily milk yield and liveweight did not differ between AA and BB animals.

CONCLUSIONS

A significant QTL determining the concentration of beta-lactoglobulin in milk was identified. Selection of animals for the beta-lactoglobulin B-allele may enable the production of milk naturally enriched for casein, thus allowing a potential increase in the yield of cheese. There may be additional future value in production of bovine milk more like human milk, where decreasing the concentration of beta-lactoglobulin is desirable.

Ryanodine receptor mutations in arrhythmia: The continuing mystery of channel dysfunction

Mutations in RyR2 are causative of an inherited disorder which often results in sudden cardiac death. Dysfunctional channel behaviour has been the subject of many investigations varying from single channel analysis through to complex animal models. This review discusses recent advances in the field, describes the controversy surrounding the exact consequences of RyR2 mutation and how the disparate data may be reconciled. This heterogeneity of function with respect to the effects of polymorphisms, phosphorylation, cytosolic and luminal Ca(2+) as well as inter-domain interactions may have important implications for the recent pharmaceutical therapies which have been put forward. We surmise that a comprehensive characterisation of mutations on a case-by-case basis may be beneficial for the development of specifically targeted therapies.

Alternative Splicing of Ryanodine Receptors Modulates Cardiomyocyte Ca 2+ Signaling and Susceptibility to Apoptosis

Ca 2+ release via type 2 ryanodine receptors (RyR2) regulates cardiac function. Molecular cloning of human RyR2 identified 2 alternatively spliced variants, comprising 30- and 24-bp sequence insertions; yet their role in shaping cardiomyocyte Ca 2+ signaling and cell phenotype is unknown. We profiled the developmental regulation and the tissue and species specificity of these variants and showed that their recombinant expression in HL-1 cardiomyocytes profoundly modulated nuclear and cytoplasmic Ca 2+ release. All splice variants localized to the sarcoplasmic reticulum, perinuclear Golgi apparatus, and to finger-like invaginations of the nuclear envelope (nucleoplasmic reticulum). Strikingly, the 24-bp splice insertion that was present at low levels in embryonic and adult hearts was essential for targeting RyR2 to an intranuclear Golgi apparatus and promoted the intracellular segregation of this variant. The amplitude variability of nuclear and cytoplasmic Ca 2+ fluxes were reduced in nonstimulated cardiomyocytes expressing both 30- and 24-bp splice variants and were associated with lower basal levels of apoptosis. Expression of RyR2 containing the 24-bp insertion also suppressed intracellular Ca 2+ fluxes following prolonged caffeine exposure (1 mmol/L, 16 hours) that protected cells from apoptosis. The antiapoptotic effects of this variant were linked to increased levels of Bcl-2 phosphorylation. In contrast, RyR2 containing the 30-bp insertion, which was abundant in human embryonic heart but was decreased during cardiac development, did not protect cardiomyocytes from caffeine-evoked apoptosis. Thus, we provide the first evidence that RyR2 splice variants exquisitely modulate intracellular Ca 2+ signaling and are key determinants of cardiomyocyte apoptotic susceptibility.

Role of ryanodine receptor mutations in cardiac pathology: more questions than answers?

The RyR (ryanodine receptor) mediates rapid Ca2+ efflux from the ER (endoplasmic reticulum) and is responsible for triggering numerous Ca2+-activated physiological processes. The most studied RyR-mediated process is excitation-contraction coupling in striated muscle, where plasma membrane excitation is transmitted to the cell interior and results in Ca2+ efflux that triggers myocyte contraction. Recently, single-residue mutations in the cardiac RyR (RyR2) have been identified in families that exhibit CPVT (catecholaminergic polymorphic ventricular tachycardia), a condition in which physical or emotional stress can trigger severe tachyarrhythmias that can lead to sudden cardiac death. The RyR2 mutations in CPVT are clustered in the N- and C-terminal domains, as well as in a central domain. Further, a critical signalling role for dysfunctional RyR2 has also been implicated in the generation of arrhythmias in the common condition of HF (heart failure). We have prepared cardiac RyR2 plasmids with various CPVT mutations to enable expression and analysis of Ca2+ release mediated by the wild-type and mutated RyR2. These studies suggest that the mutational locus may be important in the mechanism of Ca2+ channel dysfunction. Understanding the causes of aberrant Ca2+ release via RyR2 may assist in the development of effective treatments for the ventricular arrhythmias that often leads to sudden death in HF and in CPVT.

Ryanodine receptors and ventricular arrhythmias: emerging trends in mutations, mechanisms and therapies

It has been six years since the first reported link between mutations in the cardiac ryanodine receptor Ca(2+) release channel (RyR2) and catecholaminergic polymorphic ventricular tachycardia (CPVT), a malignant stress-induced arrhythmia. In this time, rapid advances have been made in identifying new mutations, and in understanding how these mutations disrupt normal channel function to cause VT that frequently degenerates into ventricular fibrillation (VF) and sudden death. Functional characterisation of these RyR2 Ca(2+) channelopathies suggests that mutations alter the ability of RyR2 to sense its intracellular environment, and that channel modulation via covalent modification, Ca(2+)- and Mg(2+)-dependent regulation and structural feedback mechanisms are catastrophically disturbed. This review reconciles the current status of RyR2 mutation-linked etiopathology, the significance of mutational clustering within the RyR2 polypeptide and the mechanisms underlying channel dysfunction. We will also review new data that explores the link between abnormal Ca(2+) release and the resultant cardiac electrical instability in VT and VF, and how these recent developments impact on novel anti-arrhythmic therapies. Finally, we evaluate the concept that mechanistic differences between CPVT and other arrhythmogenic disorders may preclude a common therapeutic strategy to normalise RyR2 function in cardiac disease.

Arrhythmogenic Mutation-Linked Defects in Ryanodine Receptor Autoregulation Reveal a Novel Mechanism of Ca 2+ Release Channel Dysfunction

Arrhythmogenic cardiac ryanodine receptor (RyR2) mutations are associated with stress-induced malignant tachycardia, frequently leading to sudden cardiac death (SCD). The causative mechanisms of RyR2 Ca 2+ release dysregulation are complex and remain controversial. We investigated the functional impact of clinically-severe RyR2 mutations occurring in the central domain, and the C-terminal I domain, a key locus of RyR2 autoregulation, on interdomain interactions and Ca 2+ release in living cells. Using high-resolution confocal microscopy and fluorescence resonance energy transfer (FRET) analysis of interaction between fusion proteins corresponding to amino- (N-) and carboxyl- (C-) terminal RyR2 domains, we determined that in resting cells, RyR2 interdomain interaction remained unaltered after introduction of SCD-linked mutations and normal Ca 2+ regulation was maintained. In contrast, after channel activation, the abnormal Ca 2+ release via mutant RyR2 was intrinsically linked to altered interdomain interaction that was equivalent with all mutations and exhibited threshold characteristics (caffeine >2.5 mmol/L; Ca 2+ >150 nmol/L). Noise analysis revealed that I domain mutations introduced a distinct pattern of conformational instability in Ca 2+ handling and interdomain interaction after channel activation that was absent in signals obtained from the central domain mutation. I domain–linked channel instability also occurred in intact RyR2 expressed in CHO cells and in HL-1 cardiomyocytes. These new insights highlight a critical role for mutation-linked defects in channel autoregulation, and may contribute to a molecular explanation for the augmented Ca 2+ release following RyR2 channel activation. Our findings also suggest that the mutational locus may be an important mechanistic determinant of Ca 2+ release channel dysfunction in arrhythmia and SCD.

Ryanodine receptor dysfunction in arrhythmia and sudden cardiac death

Mutations in ryanodine receptor calcium ion-release channels (RyR2) have emerged as important causative players in exercise/stress-induced ventricular arrhythmia leading to sudden cardiac death (SCD). Thus, RyR2 represents an attractive therapeutic target, and a detailed understanding of the mechanistic basis of RyR2 dysfunction at the molecular, cellular and organ level is essential for the development of novel, more effective therapeutic approaches to prevent arrhythmia and SCD. Such advances will translate into a tremendous improvement in the survival and quality of life of SCD-susceptible individuals. In this review, the authors consider how recent knowledge gained from mutation identification, phenotypic manifestation and functional evaluation of RyR2 mutants, are being used to develop novel therapeutic strategies in RyR2-dependent arrhythmia.

Differential Ca2+ sensitivity of RyR2 mutations reveals distinct mechanisms of channel dysfunction in sudden cardiac death

Arrhythmogenic point mutations in RyR2 result in abnormal Ca(2+) release following cardiac stimulation, leading to sudden cardiac death (SCD). Recently, we have demonstrated that significant functional differences exist between SCD-linked RyR2 mutations. Here, we investigated the molecular basis of this heterogeneity and determined the sensitivity of mutant RyR2 channels to cytoplasmic [Ca(2+)] ([Ca(2+)](c)) in living cells. Using streptolysin-O permeabilised human embryonic kidney cells, [Ca(2+)](c) was clamped in cells expressing GFP-tagged wild-type (WT) or SCD-linked RyR2 mutants (L(433)P, N(2386)I, and R(176)Q/T(2504)M). Although resting [Ca(2+)](c) was comparable in all cells, RyR2 mutants were characterised by a profound loss of Ca(2+)-dependent inhibition following caffeine stimulation when compared with WT channels. The ER Ca(2+) store was not perturbed in these experiments. Our findings support the hypothesis that SCD-linked mutational loci may be an important mechanistic determinant of RyR2 dysfunction and indicate that there is unlikely to be a unifying mechanism for channel dysfunction in SCD.

Functional heterogeneity of ryanodine receptor mutations associated with sudden cardiac death

OBJECTIVES

Point mutations in the cardiac ryanodine receptor (RyR2) mediate abnormal intracellular Ca(2+) release and are associated with stress-induced ventricular tachycardia (VT), leading to sudden cardiac death (SCD). Although the precise molecular basis of RyR2 dysfunction in SCD remains controversial, there is consensus that the mutations characterised to date all exhibit gain-of-function Ca(2+) release properties following cell stimulation. We investigated the functional impact of a distinct set of SCD-linked RyR2 mutations (L(433)P, N(2386)I, R(176)Q/T(2504)M) on intracellular Ca(2+) handling.

METHODS

We expressed full-length recombinant human wild-type (WT) and SCD-linked RyR2 mutations in human embryonic kidney (HEK) cells, and profiled the spatial and amplitude characteristics of caffeine-evoked Ca(2+) release through homo-tetrameric channels in living cells using rapid confocal laser scanning microscopy.

RESULTS

Analysis of the precise mode of Ca(2+) release in HEK cells expressing RyR2 mutants demonstrated profound differences when compared with WT channels. The SCD-linked RyR2 mutations characterised in this study exhibited heterogeneous Ca(2+) release profiles, including the novel observation that one of the mutants, (L(433)P), exhibited a marked reduction in sensitivity to channel activation. However, all SCD-linked RyR2 mutations characterised in this study resulted in an increased duration of elevated cytoplasmic Ca(2+) levels following channel activation.

CONCLUSIONS

Our live cell-based data demonstrates functional heterogeneity of Ca(2+) release through SCD-linked RyR2 mutants, suggesting that the mechanistic basis of RyR2 dysfunction in SCD may be more complex than previously anticipated. These findings may have profound consequences for the therapeutic modulation of RyR2 in stress-induced VT and SCD.

Sulfation of apomorphine by human sulfotransferases: evidence of a major role for the polymorphic phenol sulfotransferase, SULT1A1

1. The relative roles of various members of the human sulfotransferase (SULT) enzyme family in the metabolism of apomorphine, a dopamine receptor antagonist used in the treatment of Parkinson's disease and, more recently, erectile dysfunction, were examined. In humans, sulfation is the major route of metabolism of this drug. 2. Using recombinant SULTs expressed in Escherichia coli, R(--)-apomorphine sulfation was studied using the universal barium precipitation assay in the presence of [35S] 3'-phosphoadenosine 5'-phosphosulfate and SULTs 1A1, 1A2, 1A3, 1B1, 1C2, 1E1 and 2A1. It was shown that SULTs 1A1, 1A2, 1A3 and 1E1 all sulfated apomorphine to varying extents. Low activity with SULT1B1 was only seen at the highest concentration (100 microM) and no activity with SULT1C2 or SULT2A1 was observed. 3. Kinetic analysis using purified recombinant SULTs showed that 1A1, 1A3 and 1E1 all had similar Vmax/Km values, although SULT1E1 had a slightly lower Km at around 1 microM compared with approximately 4 microM for the other SULTs. 4. By correlating apomorphine sulfation (at 10 microM) in a bank of 28 liver cytosols with SULT activity towards 10 microM 4-nitrophenol (SULT1A1) and 0.2 microM 17beta-oestradiol (SULT1E1), a strong correlation with SULT1A1 activity was clearly demonstrated, suggesting this enzyme was primarily responsible for hepatic apomorphine sulfation. 5. These findings were confirmed using immuno-inhibition experiments with antibodies against SULT1A and SULT1E1, which showed preferential inhibition of apomorphine sulfation in human liver cytosol by anti-SULT1A. 6. The results strongly implicate SULT1A1 as the major enzyme responsible for hepatic apomorphine metabolism. As SULT1A1 is subject to a common functional polymorphism, sulfation phenotype may be an important determinant of susceptibility to side-effects of apomorphine and/or efficacy of treatment.

Ryanodine receptor regulation by intramolecular interaction between cytoplasmic and transmembrane domains

Ryanodine receptors (RyR) function as Ca(2+) channels that regulate Ca(2+) release from intracellular stores to control a diverse array of cellular processes. The massive cytoplasmic domain of RyR is believed to be responsible for regulating channel function. We investigated interaction between the transmembrane Ca(2+)-releasing pore and a panel of cytoplasmic domains of the human cardiac RyR in living cells. Expression of eGFP-tagged RyR constructs encoding distinct transmembrane topological models profoundly altered intracellular Ca(2+) handling and was refractory to modulation by ryanodine, FKBP12.6 and caffeine. The impact of coexpressing dsRed-tagged cytoplasmic domains of RyR2 on intracellular Ca(2+) phenotype was assessed using confocal microscopy coupled with parallel determination of in situ protein: protein interaction using fluorescence resonance energy transfer (FRET). Dynamic interactions between RyR cytoplasmic and transmembrane domains were mediated by amino acids 3722-4610 (Interacting or "I"-domain) which critically modulated intracellular Ca(2+) handling and restored RyR sensitivity to caffeine activation. These results provide compelling evidence that specific interaction between cytoplasmic and transmembrane domains is an important mechanism in the intrinsic modulation of RyR Ca(2+) release channels.

Epidemiology of insulin-using commercial motor vehicle drivers. Major variability of state licensing requirements in the U.S

OBJECTIVE

Licensing agencies in many areas, including the U.S., prohibit insulin-using individuals from driving CMVs or large trucks. This study examined the debate over the risks of licensing insulin-using individuals to drive CMVs as an occupation, and the variations in regulations of different states.

RESEARCH DESIGN AND METHODS

As part of an ongoing review of the regulations governing interstate commerce in the U.S., we surveyed all 50 states and Washington, D.C. to determine the regulations concerning intrastate driving. We received responses from 48 states and D.C., representing 95% of the U.S. population.

RESULTS

Only 9 states reported preventing insulin users from acquiring a CMV license, whereas 39 states and D.C. permitted licensing within state boundaries. Of the states allowing insulin users to drive, 26 placed special requirements on CMV licensing.

CONCLUSIONS

The results indicate that, despite a standardized U.S. federal law for driving across states, enormous variability exists in the policies for driving within states, ranging from no restrictions to a complete ban on CMV driving for insulin users.

Effect of age and sex on the composition of muscle and skin from a commercial broiler strain

The effects of age and sex have been studied on the proximate composition and inorganic constituents in breast, thigh, drumstick and skin from a commercial British broiler strain (Ross 1). Protein content generally increased with age in all muscles and decreased in skin. Protein content in skin of males was always higher than that in females. Moisture content in all muscles decreased with age. In the skin, moisture decreased with age considerably more in females than in males. Fat content increased with age in all tissues, while ash content of breast, thigh and skin decreased with age. Phosphorus, chloride, magnesium and potassium all decreased with age in all the tissues. Thigh and skin calcium content were affected by age, as was the sodium content of drumstick and skin. The sex effect on the inorganic constituents was variable.