Article

Extreme sarcoplasmic reticulum volume loss and compensatory T-tubule remodeling after Serca2 knockout

Institute for Experimental Medical Research, Oslo University Hospital and University of Oslo, 0424 Oslo, Norway.
Proceedings of the National Academy of Sciences (Impact Factor: 9.81). 03/2012; 109(10):3997-4001. DOI: 10.1073/pnas.1120172109
Source: PubMed

ABSTRACT Cardiomyocyte contraction and relaxation are controlled by Ca(2+) handling, which can be regulated to meet demand. Indeed, major reduction in sarcoplasmic reticulum (SR) function in mice with Serca2 knockout (KO) is compensated by enhanced plasmalemmal Ca(2+) fluxes. Here we investigate whether altered Ca(2+) fluxes are facilitated by reorganization of cardiomyocyte ultrastructure. Hearts were fixed for electron microscopy and enzymatically dissociated for confocal microscopy and electrophysiology. SR relative surface area and volume densities were reduced by 63% and 76%, indicating marked loss and collapse of the free SR in KO. Although overall cardiomyocyte dimensions were unaltered, total surface area was increased. This resulted from increased T-tubule density, as revealed by confocal images. Fourier analysis indicated a maintained organization of transverse T-tubules but an increased presence of longitudinal T-tubules. This demonstrates a remarkable plasticity of the tubular system in the adult myocardium. Immunocytochemical data showed that the newly grown longitudinal T-tubules contained Na(+)/Ca(2+)-exchanger proximal to ryanodine receptors in the SR but did not contain Ca(2+)-channels. Ca(2+) measurements demonstrated a switch from SR-driven to Ca(2+) influx-driven Ca(2+) transients in KO. Still, SR Ca(2+) release constituted 20% of the Ca(2+) transient in KO. Mathematical modeling suggested that Ca(2+) influx via Na(+)/Ca(2+)-exchange in longitudinal T-tubules triggers release from apposing ryanodine receptors in KO, partially compensating for reduced SERCA by allowing for local Ca(2+) release near the myofilaments. T-tubule proliferation occurs without loss of the original ordered transverse orientation and thus constitutes the basis for compensation of the declining SR function without structural disarrangement.

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    • "Approaches to determine the integrity of the T-tubule system include measuring the density (area) (He et al., 2001; Heinzel et al., 2008; Lyon et al., 2009; Pavlovic et al., 2010; Kemi et al., 2011; Ibrahim et al., 2013) and regularity based on the power of the frequency spectrum of spatially repeated elements (TT power ). (Song et al., 2006; Wei et al., 2010; Kemi et al., 2011; Wu et al., 2011; Swift et al., 2012; Ibrahim et al., 2013). However, caveats that are unique to each of these two methods limit data interpretation and quantitative comparison of images acquired from different studies. "
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    Frontiers in Physiology 05/2015; 6. DOI:10.3389/fphys.2015.00134 · 3.50 Impact Factor
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    • "Indeed, NCX upregulation is a common feature of diseased cardiomyocytes (Ottolia et al., 2013) (Figure 2). Interestingly, dyadic junctions are formed between newly grown longitudinal tubules and SR, with co-localization of NCX and RyRs (Swift et al., 2012). Based on these data, mathematical modelling suggested a role for NCX-triggered Ca 2+ release at these sites. "
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    • "Collectively, these changes prompt the hypothesis that Serca2 KO cardiac performance in vivo is maintained by enhancing trans-sarcolemmal Ca2+ transport while SR Ca2+ handling is reduced. KO myocytes exhibit greater Ca2+ transients than FL when stimulated during caffeine perfusion[18]. The contractile performance of isolated KO hearts perfused with caffeine, however, suggests that trans-sarcolemmal Ca2+ flux is unlikely to account for preserved systolic function after Serca2 disruption. "
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