Background:
The cardiac dyad and associated calcium (Ca
2+
) release units (CRUs) are integral for cardiac excitation-contraction (E-C) coupling. The cardiac dyad is a multicomponent complex composed of the transverse-tubules, junctional sarcoplasmic reticulum (jSR), and CRU-associated proteins. Proper E-C coupling requires Ca
2+
influx via long-lasting-L-type Ca
2+
channels (Cav1.2), which triggers Ca
2+
release from adjacent ryanodine receptor 2 (RyR2) located in jSR. βII-spectrin loss alters RyR2 localization, but its contribution to cardiac dyad organization and E-C coupling remains unclear.
Hypothesis:
We hypothesized that βII-spectrin is required for cardiac dyad organization and proper cardiomyocyte calcium signaling.
Methods:
Cardiomyocyte-specific βII-spectrin knockout (βII-cKO) and control mice (βII-flox) were generated and harvested for immunoblotting, qRT-PCR, and imaging by transmission electron microscopy (TEM). Mice hearts were also interrogated by echocardiography following transaortic constriction.
Results:
Cardiac dyad structure and CRU components are significantly altered in βII-cKO hearts. Ultrastructural analysis of βII-cKOhearts by TEM showed fragmented cardiac dyads, dilated jSR and reduced jSR contact with T-tubules. Quantitative RT-PCR showed increased expression of calsequestrin 1 and 2, decreased phospholambam,
Ncx
and
Cacna1c
(major Cav1.2 subunit), while
Ryr2
,
Jph2
,
Serca2a
are unchanged. Protein expression of NCX was decreased, while expression of CAV1.2, RYR2, CASQ2, and SERCA2A remained unchanged. Furthermore, pressure-induced overload of βII-cKO hearts promoted accelerated heart failure (HF).
Conclusions:
This is the first demonstration that the βII-spectrin cytoskeletal protein is critically important for maintaining the structural integrity of the cardiac dyad. βII-spectrin loss and the resulting alterations in CRU structure, genes, and protein composition promoting accelerated HF.
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