Dyad content is reduced in cardiac myocytes of mice with impaired calmodulin regulation of RyR2

Abstract

In cardiac muscle, calmodulin (CaM) regulates the activity of several membrane proteins involved in Ca(2+) homeostasis (CaV1.2; RyR2, SERCA2, PMCA). Three engineered amino acid substitutions in the CaM binding site of the cardiac ryanodine receptor (RyR2) in mice (Ryr2 (ADA/ADA) ) strongly affect cardiac function, with impaired CaM inhibition of RyR2, reduced SR Ca(2+) sequestration, and early cardiac hypertrophy and death (Yamaguchi et al., J Clin Invest 117:1344-1353, 2007). We have examined the ultrastructure and RyR2 immunolocalization in WT and Ryr2 (ADA/ADA) hearts at ~10 days after birth. The myocytes show only minor evidence of structural damage: some increase in intermyofibrillar space, with occasional areas of irregular SR disposition and an increase in frequency of smaller myofibrils, despite an increase of about 15 % in average myocyte cross sectional area. Z line streaming, a sign of myofibrillar stress, is limited and fairly rare. Immunolabeling with an anti-RyR2 antibody shows that RyR-positive foci located at the level of the Z lines are less frequent in mutant hearts. A dramatic decrease in the frequency and size of dyads, accompanied by a decrease in occupancy of the gap by RyR2, but without obvious alterations in location and general structure is a notable ultrastructural feature. The data suggest that the uneven distribution of dyads or calcium release sites within the cells resulting from an overall reduction in RyR2 content may contribute to the poor cardiac performance and early death of Ryr2 (ADA/ADA) mice. An unusual fragmentation of mitochondria, perhaps related to imbalances in free cytoplasmic calcium levels, accompanies these changes.

BIN1 regulates dynamic t-tubule membrane

“…By promoting actin polymerization through N-WASP activation, BIN1+13+17 anchors and maintains these t-tubule microfolds to myofilament Z-disc structural protein α-actinin. Using transmission electron microscopy imaging, Dr. Franzini-Armstrong’s laboratory also observed t-tubule membrane infolding to lumen at t-tubule cross sections from mouse hearts [105], similar to the t-tubule microfolds reported in our study. These microfolds help trap extracellular ions, which create a diffusion barrier inside the t-tubule network.…”

Elongated mitochondrial constrictions and fission in muscle fatigue

“…If mitochondria undergo fission, the obvious question is whether this fission results in the appearance of small mitochondrial profiles deriving from fragmentation of the normal elongated mitochondria. Mitochondrial fragmentation, resulting in the accumulation of smaller organelles, is easily detected in conditions of energetic stress and unbalanced Ca 2+ homeostasis (Wu et al, 2010;Iqbal and Hood, 2014;Lavorato et al, 2015). However, despite evidence for possible mitochondrial fission events, we find very little obvious evidence for fragmented mitochondria after electrical stimulation and practically no evidence after treadmill exercise.…”

“…Long-range effects occur in myopathies affecting calcium homeostasis, such as central core disease and malignant hyperthermia in which mitochondrial morphology, distribution and fragmentation are strongly affected (Boncompagni et al, 2009b;Lavorato et al, 2016). Prolonged unbalanced Ca 2+ homeostasis in the myocardium activates the fission process, resulting in increased frequency of mitochondrial profiles due to fragmentation, while also inducing prolonged nanotunneling extensions in the usually structurally conservative cardiac mitochondria (Lavorato et al, 2015(Lavorato et al, , 2017.…”

Cardiovascular consequences of metabolic syndrome

“…Investigators in this field have identified relevant and important molecular pathways linking obesity to cardiac dysfunction. In particular, alterations in myocardial Ca 2+ handling via changes in the functional expression of SERCA2A and ryanodine (RyR2) receptors(52, 57, 64) have been of interest, including a recent comprehensive paper demonstrating concurrent effects of RyR2 abnormalities to induced myocellular and β-cell dysfunction (65), affecting subcellular structures in addition to producing abnormalities in intracellular Ca 2+ handling (66). This shared dependence of the myocardium and β-cell highlights the need for further studies in to the mechanisms underlying the dysfunctional regulation of Ca 2+ in obesity/MetS.…”