Junctophilin-2 is necessary for T-tubule maturation during mouse heart development

Journal of Cell Science

et al. 2016

Self Cite

Signalling nanodomains requiring close contact between the plasma membrane and internal compartments, known as ‘junctions’, are fast communication hubs within excitable cells such as neurones and muscle. Here we have examined two transgenic murine models probing the role of junctophilin-2, a membrane tethering protein crucial for the formation and molecular organisation of sub-microscopic junctions in ventricular muscle cells of the heart. Quantitative single molecule localisation microscopy showed that junctions in animals producing above-normal levels of junctophilin-2 were enlarged, allowing the re-organisation of the primary functional protein within it, the ryanodine receptor (RyR). Although this change was associated with much enlarged RyR clusters that due to their size should be more excitable, functionally it caused a mild inhibition in the calcium signalling output of the junctions (calcium sparks). Analysis of the single molecule densities of both RyR and junctophilin-2 revealed an ∼3-fold increase in the junctophilin-2 to RyR ratio. This molecular rearrangement is compatible with direct inhibition of RyR opening by junctophilin-2 to intrinsically stabilise the calcium signalling properties of the junction and thus the contractile function of the cell.

Section: Discussionmentioning

confidence: 69%

Junctophilin-2 in the nanoscale organisation and functional signalling of ryanodine receptor clusters in cardiomyocytes

Munro

Jayasinghe

Journal of Cell Science

et al. 2016

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“…Initially, JPH2 was reported to function as a tether that connects the PM to the SR within the cardiac dyad (19,23). Recent studies, however, have also demonstrated that JPH2 acts as a critical regulator of cardiomyocyte development by promoting the maturation of transverse tubules (21). In addition, JPH2 was shown to modulate intracellular Ca 2ϩ handling through regulation of RyR2 (6,25).…”

Section: Discussionmentioning

confidence: 99%

“…The loss of JPH2 was shown to increase variability in the distance between the PM and SR, whereas the mean distance remained unaltered. Moreover, JPH2 has also been shown to directly bind to and inhibit RyR2 function, suggesting that JPH2 might have nonstructural roles in cardiomyocytes (11,21). Cardiac-specific conditional knockdown of JPH2 resulted in increased RyR2 sparking and channel opening with resultant decrease in CICR and ECC gain (27).…”

mentioning

confidence: 99%

Reduced junctional Na⁺/Ca²⁺-exchanger activity contributes to sarcoplasmic reticulum Ca²⁺ leak in junctophilin-2-deficient mice

Landstrom

American Journal of Physiology-Heart and Circulatory Physiology

et al. 2014

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Expression silencing of junctophilin-2 (JPH2) in mouse heart leads to ryanodine receptor type 2 (RyR2)-mediated sarcoplasmic reticulum (SR) Ca2+ leak and rapid development of heart failure. The mechanism and physiological significance of JPH2 in regulating RyR2-mediated SR Ca2+ leak remains elusive. We sought to elucidate the role of JPH2 in regulating RyR2-mediated SR Ca2+ release in the setting of cardiac failure. Cardiac myocytes isolated from tamoxifen-inducible conditional knockdown mice of JPH2 (MCM-shJPH2) were subjected to confocal Ca2+ imaging. MCM-shJPH2 cardiomyocytes exhibited an increased spark frequency width with altered spark morphology, which caused increased SR Ca2+ leakage. Single channel studies identified an increased RyR2 open probability in MCM-shJPH2 mice. The increase in spark frequency and width was observed only in MCM-shJPH2 and not found in mice with increased RyR2 open probability with native JPH2 expression. Na+/Ca2+-exchanger (NCX) activity was reduced by 50% in MCM-shJPH2 with no detectable change in NCX expression. Additionally, 50% inhibition of NCX through Cd2+ administration alone was sufficient to increase spark width in myocytes obtained from wild-type mice. Additionally, superresolution analysis of RyR2 and NCX colocalization showed a reduced overlap between RyR2 and NCX in MCM-shJPH2 mice. In conclusion, decreased JPH2 expression causes increased SR Ca2+ leakage by directly increasing open probability of RyR2 and by indirectly reducing junctional NCX activity through increased dyadic cleft Ca2+. This demonstrates two novel and independent cellular mechanisms by which JPH2 regulates RyR2-mediated SR Ca2+ leak and heart failure development.

“…Caveonlin-3 organized caveolae distributes along t-tubules and caveolae distribution is altered in Bin1-deleted mouse cardiomyocytes (Laury-Kleintop et al, 2015), indicating crosstalk between cBIN1-microdomains and caveolae microdomains within t-tubules. JPH2 is a t-tubule/jSR junctional protein and is involved in the maintenance of junctional membrane structure, which when knocked down increases the distance between t-tubule and jSR membrane and disrupts jSR membrane complexes (van Oort et al, 2011;Wu et al, 2012;Reynolds et al, 2013). A role of JPH2 in HF progression has been reported (see review in (Takeshima et al, 2015)).…”

Section: The Role Of Bin1 In T-tubule Microdomain Organization and Fumentioning

confidence: 99%

Cardiac BIN1 (cBIN1) is a regulator of cardiac contractile function and an emerging biomarker of heart muscle health

Zhou

Hong

2016

Sci. China Life Sci.

In recent decades, a cardiomyocyte membrane scaffolding protein bridging integrator 1 (BIN1) has emerged as a critical multifunctional regulator of transverse-tubule (t-tubule) function and calcium signaling in cardiomyocytes. Encoded by a single gene with 20 exons that are alternatively spliced, more than ten BIN1 protein isoforms are expressed with tissue and disease specificity. The recently discovered cardiac alternatively spliced isoform BIN1 (cBIN1 or BIN1+13+17)plays a crucial role in organizing membrane microfolds within cardiac t-tubules. These cBIN1-induced microfolds form functional dyad microdomains by trafficking L-type calcium channels (LTCC) to t-tubule membrane and recruiting ryanodine receptors (RyR) to junctional sarcoplasmic reticulum membrane. When cBIN1 is transcriptionally reduced as occurs in heart failure, cBIN1-microfolds are disrupted and fail to form LTCC and RyR couplons. As a result, impaired dyad formation limits excitation-contraction coupling thus cardiac contractility, and accumulation of orphaned leaky RyRs outside of dyads increases ventricular arrhythmias. Reduced myocardial BIN1 in heart failure is also detectable at the blood level, and plasma BIN1 level predicts heart failure progression and future arrhythmias in cardiomyopathy patients. Here we will review the recent progress in BIN1-related cardiomyocyte biology studies and discuss the diagnostic and predictive values of cBIN1 in future clinical use. heart failure, cBIN1, t-tubules, calcium transient, arrhythmias Citation:Zhou, K., and Hong, T. (2017). Cardiac BIN1 (cBIN1) is a regulator of cardiac contractile function and an emerging biomarker of heart muscle health.