Dataset Information

Junctional membrane Ca2+ dynamics in human muscle fibers are altered by malignant hyperthermia causative RyR mutation.

ABSTRACT: We used the nanometer-wide tubules of the transverse tubular (t)-system of human skeletal muscle fibers as sensitive sensors for the quantitative monitoring of the Ca2+-handling properties in the narrow junctional cytoplasmic space sandwiched between the tubular membrane and the sarcoplasmic reticulum cisternae in single muscle fibers. The t-system sealed with a Ca2+-sensitive dye trapped in it is sensitive to changes in ryanodine receptor (RyR) Ca2+ leak, the store operated calcium entry flux, plasma membrane Ca pump, and sodium-calcium exchanger activities, thus making the sealed t-system a nanodomain Ca2+ sensor of Ca2+ dynamics in the junctional space. The sensor was used to assess the basal Ca2+-handling properties of human muscle fibers obtained by needle biopsy from control subjects and from people with a malignant hyperthermia (MH) causative RyR variant. Using this approach we show that the muscle fibers from MH-susceptible individuals display leakier RyRs and a greater capacity to extrude Ca2+ across the t-system membrane compared with fibers from controls. This study provides a quantitative way to assess the effect of RyR variants on junctional membrane Ca2+ handling under defined ionic conditions.

SUBMITTER: Cully TR

PROVIDER: S-EPMC6094124 | biostudies-literature | 2018 Aug

REPOSITORIES: biostudies-literature

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Junctional membrane Ca2+ dynamics in human muscle fibers are altered by malignant hyperthermia causative RyR mutation.

Cully Tanya R TR Choi Rocky H RH Bjorksten Andrew R AR Stephenson D George DG Murphy Robyn M RM Launikonis Bradley S BS

Proceedings of the National Academy of Sciences of the United States of America 20180723 32

We used the nanometer-wide tubules of the transverse tubular (t)-system of human skeletal muscle fibers as sensitive sensors for the quantitative monitoring of the Ca2+-handling properties in the narrow junctional cytoplasmic space sandwiched between the tubular membrane and the sarcoplasmic reticulum cisternae in single muscle fibers. The t-system sealed with a Ca2+-sensitive dye trapped in it is sensitive to changes in ryanodine receptor (RyR) Ca2+ leak, the st ...[more]

PMID: 30038012

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Project description:BackgroundVariants in RYR1, the gene encoding the ryanodine receptor-1, can give rise to a wide spectrum of neuromuscular conditions. Muscle imaging abnormalities have been demonstrated in isolated cases of patients with a history of RYR1-related malignant hyperthermia (MH) susceptibility.ObjectiveTo provide insights into the type and prevalence of muscle ultrasound abnormalities and muscle hypertrophy in patients carrying gain-of-function RYR1 variants associated with MH susceptibility and to contribute to delineating the wider phenotype, optimizing the diagnostic work-up and care for MH susceptible patients.MethodsWe performed a prospective cross-sectional observational muscle ultrasound study in patients with a history of RYR1-related MH susceptibility (n = 40). Study procedures included a standardized history of neuromuscular symptoms and a muscle ultrasound assessment. Muscle ultrasound images were analyzed using a quantitative and qualitative approach and compared to reference values and subsequently subjected to a screening protocol for neuromuscular disorders.ResultsA total of 15 (38%) patients had an abnormal muscle ultrasound result, 4 (10%) had a borderline muscle ultrasound screening result, and 21 (53%) had a normal muscle ultrasound screening result. The proportion of symptomatic patients with an abnormal result (11 of 24; 46%) was not significantly higher compared to the proportion of asymptomatic patients with an abnormal ultrasound result (4 of 16; 25%) (P = 0.182). The mean z-scores of the biceps brachii (z = 1.45; P < 0.001), biceps femoris (z = 0.43; P = 0.002), deltoid (z = 0.31; P = 0.009), trapezius (z = 0.38; P = 0.010) and the sum of all muscles (z = 0.40; P < 0.001) were significantly higher compared to 0, indicating hypertrophy.ConclusionsPatients with RYR1 variants resulting in MH susceptibility often have muscle ultrasound abnormalities. Frequently observed muscle ultrasound abnormalities include muscle hypertrophy and increased echogenicity.

Project description:In cardiac myocytes, clusters of type-2 ryanodine receptors (RyR2s) release Ca2+ from the sarcoplasmic reticulum (SR) via a positive feedback mechanism in which fluxed Ca2+ activates nearby RyRs. Although the general principles of this are understood, less is known about how single-RyR gating properties define the RyR group dynamics in an array of many channels. Here, we examine this using simulations with three models of RyR gating that have identical open probabilities: the commonly used two-state Markov gating model, one that utilizes multiple exponentials to fit single-channel open time (OT) and closed time (CT) distributions, and an extension of this multiexponential model that also includes experimentally measured correlations between single-channel OTs and CTs. The simulations of RyR clusters that utilize the multiexponential gating model produce infrequent Ca2+ release events with relatively few open RyRs. Ca2+ release events become even smaller when OT/CT correlations are included. This occurs because the correlations produce a small but consistent bias against recruiting more RyRs to open during the middle of a Ca2+ release event, between the initiation and termination phases (which are unaltered compared to the uncorrelated simulations). In comparison, the two-state model produces frequent, large, and long Ca2+ release events because it had a recruitment bias in favor of opening more RyRs. This difference stems from the two-state model's single-RyR OT and CT distributions being qualitatively different from the experimental ones. Thus, the details of single-RyR gating can profoundly affect SR Ca2+ release even if open probability and mean OTs and CTs are identical. We also show that Ca2+ release events can terminate spontaneously without any reduction in SR [Ca2+], luminal regulation, Ca2+-dependent inactivation, or physical coupling between RyRs when Ca2+ flux is below a threshold value. This supports and extends the pernicious attrition/induction decay hypothesis that SR Ca2+ release events terminate below a threshold Ca2+ flux.

Dataset Information

Junctional membrane Ca2+ dynamics in human muscle fibers are altered by malignant hyperthermia causative RyR mutation.

Publications

Junctional membrane Ca<sup>2+</sup> dynamics in human muscle fibers are altered by malignant hyperthermia causative RyR mutation.

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