Three-dimensional distribution of ryanodine receptor clusters in cardiac myocytes

Abstract

The clustering of ryanodine receptors (RyR2) into functional Ca2+ release units is central to current models for cardiac excitation-contraction (E-C) coupling. Using immunolabeling and confocal microscopy, we have analyzed the distribution of RyR2 clusters in rat and ventricular atrial myocytes. The resolution of the three-dimensional structure was improved by a novel transverse sectioning method as well as digital deconvolution. In contrast to earlier reports, the mean RyR2 cluster transverse spacing was measured 1.05 microm in ventricular myocytes and estimated 0.97 microm in atrial myocytes. Intercalated RyR2 clusters were found interspersed between the Z-disks on the cell periphery but absent in the interior, forming double rows flanking the local Z-disks on the surface. The longitudinal spacing between the adjacent rows of RyR2 clusters on the Z-disks was measured to have a mean value of 1.87 microm in ventricular and 1.69 microm in atrial myocytes. The measured RyR2 cluster distribution is compatible with models of Ca2+ wave generation. The size of the typical RyR2 cluster was close to 250 nm, and this suggests that approximately 100 RyR2s might be present in a cluster. The importance of cluster size and three-dimensional spacing for current E-C coupling models is discussed.

FIGURE 1

Ryanodine receptors (RyR2) in cardiac myocytes were labeled using an anti-RyR2 monoclonal antibody and visualized using a secondary antibody conjugated Alexa Fluor 488. Images show examples of RyR2 labeling in (A) an isolated rat atrial myocyte, (B) left ventricle tissue cross section, (C) an isolated rat atrial myocyte, and (D) right atrium tissue cross section. The scale bar is 2.0 μm. The longitudinal spacing between the adjacent rows of RyR2 clusters was compared with the sarcomere length in live cells. (E) Transmission image of a typical freshly isolated rat ventricular myocyte shows clear sarcomeres. (F) The RyR2 cluster longitudinal spacing in isolated cells (shown as mean ± SE) is essentially the same as the sarcomere length in live cells. Furthermore, the RyR2 spacing in the isolated cells remains similar to that in the tissue cross section (t-test, p = 0.14).

FIGURE 2

Distance between adjacent RyR2 clusters from longitudinal section images. Panel A shows the three ROIs that were measured to obtain the longitudinal, the transverse, and the peripheral RyR2 cluster spacing, respectively. (B) Intensity profiles along each ROI. The stars show the peaks of intensity, and their distance is measured as the spacing between the RyR2 clusters. (C) Histograms of RyR2 spacing in atrial and ventricular myocytes. The mean and the standard deviation values are listed in Table 1.

FIGURE 3

Analysis of transverse RyR2 cluster spacing. Panel A shows the conventional orientation of confocal sectioning with respect to the cell axis. Panel B shows a typical cross section. Note the points of dislocation (arrows) and the curved distribution of RyR2 labeling. Panel C shows how 3-D distribution of RyR2 clusters, when combined with an extended section resolution, leads to an overestimate of RyR2 density. The long ellipses represent optically blurred RyR2 clusters. D shows the cell orientation used in the agarose embedding method. In this orientation, resolution of the transverse section is increased to that of the microscope's in-plane resolution. Panel E shows a typical cell cross section when the cell is oriented as shown in D. Arrows A and B show regions analyzed in panel F. Note the phase difference between the maxima of labeling intensity as the confocal section is moved along the cell. The mean distance between peaks is 2.1 ± 0.7 μm and 1.9 ± 0.6 μm for site A and B, respectively, which is in agreement with the longitudinal RyR2 spacing value obtained from the conventional longitudinal section method as shown in Fig. 1. See the movie in Supplementary Materials for an animated display of the RyR2 labeling in transverse section images along the z axis. Panel G shows image processing with a new digital filter to define the center of RyR2 cluster labeling (see Methods and Materials). The original image, the binary image after thresholding, and the density-dependent filtered image are displayed in order from left to right. In the filtered image, the in-focus RyR2 labeling is picked up and the background noise is reduced. H shows the histogram of RyR2 cluster nearest neighbor distances in the transverse plane. The mean and standard deviation are 1.05 ± 0.44 μm with a median value of 0.96 μm for the transverse spacing. Panel I shows the curvature of RyR2 cluster distribution in the transverse section. Note how the position of the wave moves across the cell with section position. Scale bar 5.0 μm.

FIGURE 4

Test of filter algorithm on a virtual cell. The top of panel A shows the transverse sectioning geometry of a model cell. The middle panel shows evenly spaced sections through the simulated data. The randomly placed pixels were set to appear as bright spots whose mean nearest neighbor distance was set to 1.0 μm. The lower panel shows the simulated confocal sections after blurring by convolving with the microscope PSF and adding noise. Note the bright spots appearing only on Z-lines in the unblurred myocyte (middle panel) are now seen on subsequent image planes in the blurred images (bottom panel). After processing these data with the algorithm used in Fig. 3, the mean nearest neighbor distance was 1.0 ± 0.2 μm and the median distance was 1.0 μm, as expected. Panel B shows simulated longitudinal section images generated from the same data set. The measured transverse spacing was 0.88 μm, which is shorter than the known set value of 1.0 μm, illustrating the artifactual reduction in transverse spacing seen in the longitudinal section.

FIGURE 5

RyR2 cluster distribution within the cell interior and the periphery. Panel A shows sequential z-section images separated by 0.2 μm. A movie showing a z-section image stack is attached in the Supplementary Materials. After processing these data by 3-D image deconvolution using the microscope PSF, B shows sections obtained across the cell (left panel) and on the cell surface (middle panel). The autocorrelation of the surface image (right panel) reveals the periodicities in the RyR2 labeling. There is a strong correlation (upper red arrow) at ∼2 μm along the longitudinal axis and at ∼1 μm along the transverse axis (labeled Radial) in correspondence with the measured longitudinal and transverse spacing. An additional peak (lower red arrow) also occurs at ∼0.6 μm and 0.8 μm at longitudinal and transverse axes, respectively. This extra peak reflects the mean position of the intercalated peripheral RyR2 clusters, which are loosely arrayed with this spacing in double rows (middle panel). The overlaid red curves show the autocorrelations at L = 0 and R = 0. Panel C shows a schematic of the 3-D geometry of RyR2 cluster distribution in cardiac myocytes as used in the mathematical model presented in the companion work (Izu et al., 2005).

FIGURE 6

High resolution deconvolved image of RyR2 cluster size. The panel at left shows that most clusters appear punctate with size close to the resolution limit (∼240 nm). Occasional clusters were larger, and two examples of such large clusters are shown here (arrows in right panel). One has a maximum diameter of 410 nm and the curved cluster has a length of 960 nm. The extensive size is not due to out-of-focus labeling because the maximum labeling intensity is contained within this optical section. Scale bars are 2.0 μm (left panel) and 200 nm (right panel).