Edwin D Moore

Single-molecule localization microscopy has greatly improved our understanding of biology by providing super-resolution images of biological processes and structures. However, it is still very challenging to apply this technique to thick tissues. A 3D imaging system based on single-molecule localization microscopy is presented to allow high-accuracy drift-free (
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The focus of this thesis is to address the location and distribution of the type 2 Ryanodine Receptor (RyR2) in mammalian cardiac myocytes with respect to their function. These integral membrane proteins function as Ca²⁺-activated Ca²⁺ ion channels as well as a scaffold for a large number of signaling molecules that modulate the release of Ca²⁺ through the channel. The relative position of the RyR2 tetramers is therefore a critical determinant of their function. To study this question, I have used a combination of immunofluorescence microscopy, transmission electron microscopy, and tomography to map the position of the tetramers in whole cells and in cell sections and have used tissue obtained from both rat and human hearts. Biochemical and physiological techniques were used to correlate structure with function.I have found that RyR2s are located only in three regions: in couplons on the surface, transverse tubules and on most of the axial tubules. In all regions, most but not all of the RyR2s colocalize with the voltage-gated Ca²⁺ channel (Cav1.2), suggesting that they play a role in excitation-contraction coupling. Some RyR2 are colocalized with cavelin-3 and not with Cav1.2 and hypothesized that these ‘extra-couplonic’ RyR2 might be regulated by the multitude of signaling molecules associated with caveolin-3 to modulate Ca²⁺ release. Dual-tilt electron tomography produced en face views of both rat and human dyads, enabling a direct examination of RyR2 arrangement. Both species showed that tetramer packing was non-uniform containing a mix of checkerboard and side-by-side arrangements as well as isolated tetramers. Finally, I showed that the tetramers’ arrangement depended on the Mg²⁺ concentration and on their phosphorylation status; in low Mg²⁺ and after phosphorylation RyR2s were positioned in largely checkerboard arrangements while in response to high Mg²⁺ the tetramers were positioned largely side by side. These tetramer arrangements: side by side, mixed and checkerboard were associated with progressively increasing spark frequencies. The correlation between tetramer arrangement and spark frequency suggests that tetramer rearrangement may be another mechanism whereby physiological processes operate and provides potential new mechanisms by which the activity of RYR2, the dyad and cardiac contractility may be regulated.

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Excitation-contraction (EC) coupling in the neonatal rabbit heart has been previously shown to be mediated predominately by reverse-mode activity of the sodium-calcium exchanger (NCX). Thus the regulation of NCX is a primary determinant of neonatal cardiac contractility. It is proposed that in neonate hearts, a restricted domain allows a sodium current (INa) to mediate a large elevation in subsarcolemmal sodium concentration which then drives calcium entry through reverse-mode NCX. Functional data suggest that calcium influx through NCX can also trigger calcium induced calcium release (CICR).Traditionally, neonatal myocytes are thought to mediate EC coupling exclusively through trans-sarcolemma calcium influx. This model of EC coupling is distinct from the adult model of EC coupling in that it does not involve a significant CICR component. Traditionally, CICR processes are thought to be a hallmark of adult EC coupling processes where CICR is triggered exclusively by the L-type calcium current. Neonatal myocytes were previously believed to be too immature to sustain physiologically significant levels of CICR. Yet recent functional data suggest that not only are neonatal myocytes able to sustain CICR but that neonatal myocytes trigger CICR independently of the calcium current. Neonatal myocytes appear to trigger CICR exclusively though reverse-mode NCX activity (NCX-CICR).The phenomenon of NCX-CICR, prominent in early developmental stages and declining with further development, suggest that the neonatal myocardium contains specialized microdomains that allow NCX-CICR to occur. To investigate this unique EC coupling phenotype, three-dimensional confocal microscopy and advanced digital image analysis techniques are utilized to quantify the presence of these specialized microdomains and to determine the changes in these microdomains that occur with development.

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