Real-time intra-store confocal Ca2+ imaging in isolated mouse cardiomyocytes

Impaired Binding to Junctophilin-2 and Nanostructural Alteration in CPVT Mutation

[Figure: see text].

Simultaneous Recording of Subcellular Ca2+ Signals from the Cytosol and Sarco/Endoplasmic Reticulum: Compartmentalized Dye Loading, Imaging, and Analysis

An increase in the cytosolic Ca²⁺ concentration triggers the contraction in cardiomyocytes. In these cells sarcoplasmic reticulum (SR) is the major source of Ca²⁺, and the release from this store is mediated by the ryanodine receptors (RyRs). These receptors are regulated by cytosolic and intra-SR [Ca²⁺]. The cytosolic Ca²⁺ regulation is well established, but there are some limitations to determine indirectly the intra-SR Ca²⁺ concentration and understand its role in the RyRs regulation. Therefore, the interest to directly measure the free intra-SR Ca²⁺ concentration ([Ca²⁺]_SR) has led to the application of a low-affinity Ca²⁺ indicator (Fluo-5N AM) to follow changes of [Ca²⁺]_SR in cardiomyocytes. However the loading of this AM-ester dye into the SR has remained a challenge in freshly isolated mouse cardiomyocytes. Here, we describe an optimized protocol to measure changes of [Ca²⁺]_SR in mouse cardiomyocytes using fluorescent Ca²⁺ indicators and confocal microscopy. The application of this protocol allows to evaluate directly intra-SR Ca²⁺ in real time in various mouse models of cardiac disease, including transgenic animals harboring mutants of RyRs or other Ca²⁺ signaling proteins.

Automatic Detection and Classification of Ca2+ Release Events in Line- and Frame-Scan Images

Analysis of Ca²⁺ signals obtained in various cell types (i.e., cardiomyocytes) is always a tradeoff between acquisition speed and signal/noise ratio of the fluorescence signal. This becomes especially apparent during fast two- or three-dimensional confocal imaging when local intracellular fluorescence signals originating from Ca²⁺ release from intracellular Ca²⁺ stores (e.g., sarcoplasmic reticulum) need to be examined. Mathematical methods have been developed to remedy a high noise level by fitting each pixel with a transient function to "denoise" the image. So far, current available analytical approaches have been impaired by a number of constraints (e.g., inability to fit local, concurrent, and consecutive events) and the limited ability to customize implementation. Here, we suggest a, to our knowledge, novel approach for detailed analysis of subcellular micro-Ca²⁺ events based on pixel-by-pixel denoising of confocal frame- and line-scan images. The algorithm enables spatiotemporally overlapping events (e.g., a Ca²⁺ spark occurring during the decaying phase of a Ca²⁺ wave) to be extracted so that various types of Ca²⁺ events can be detected at a pixel time level of precision. The method allows a nonconstant baseline to be estimated for each pixel, foregoing the need to subtract fluorescence background or apply self-ratio methods before image analysis. Furthermore, by using a clustering algorithm, identified single-pixel events are grouped into "physiologically relevant" Ca²⁺ signaling events spanning multiple pixels (sparks, waves, puffs, transients, etc.), from which spatiotemporal event parameters (e.g., full duration at half maximal amplitude, full width at half maximal amplitude, amplitude, wave speed, rise, and decay times) can be easily extracted. The method was implemented with cross-platform open source software, providing a comprehensive and easy-to-use graphical user interface enabling rapid line-scan images and rapid frame-scan image sequences (up to 150 frames/s) to be analyzed and repetitive Ca²⁺ events (Ca²⁺ sparks and Ca²⁺ puffs) originating from clusters of Ca²⁺ release channels located in the sarcoplasmic reticulum membrane (ryanodine receptors and inositol 1,4,5-trisphosphate receptors) of isolated cardiomyocytes to be examined with a high level of precision.

Stabilization of Ca2+ signaling in cardiac muscle by stimulation of SERCA

AIMS

In cardiac muscle, phosphorylation of the RyRs is proposed to increase their Ca²⁺ sensitivity. This mechanism could be arrhythmogenic via facilitation of spontaneous Ca²⁺ waves. Surprisingly, the level of Ca²⁺ inside the SR needed to initiate such waves has been reported to increase upon β-adrenergic stimulation, an observation which cannot be easily reconciled with elevated Ca²⁺ sensitivity of the RyRs. We tested the hypothesis that this change of Ca²⁺ wave threshold could occur indirectly, subsequent to SERCA stimulation.

METHODS AND RESULTS

Cytosolic and intra-SR Ca²⁺ waves were simultaneously recorded with confocal line-scan imaging in intact and permeabilized mouse cardiomyocytes using Rhod-2 and Fluo-5-N, respectively. We analyzed changes of several Ca²⁺ signaling parameters during specific SERCA stimulation by ochratoxin A (OTA), jasmonate or the Fab fragment of a phospholamban antibody. SERCA stimulation resulted in a substantial increase of the threshold for Ca²⁺ wave initiation. Faster Ca²⁺ transient decay and SR refilling confirmed SERCA acceleration.

CONCLUSIONS

These results suggest that isolated SERCA stimulation can elevate the intra-SR threshold for the generation of Ca²⁺ waves, independently of RyR phosphorylation. Simultaneously, fractional Ca²⁺ release and wave amplitudes are reduced. Thus, SERCA stimulation appears to exert a negative feed-back on the Ca²⁺-induced Ca²⁺ release mechanisms sustaining the waves. Thereby, it may be profoundly antiarrhythmic. This may be clinically relevant when therapies are applied to stimulate the SERCA activity (e.g. SERCA overexpression with gene therapy, future small molecule SERCA stimulators).