Optical interrogation of sympathetic neuronal effects on macroscopic cardiac monolayer dynamics

Abstract

Alterations in autonomic function are known to occur in cardiac conditions including sudden cardiac death. Cardiac stimulation via sympathetic neurons can potentially trigger arrhythmias. Dissecting direct neural-cardiac interactions at the cellular level is technically challenging and understudied due to the lack of experimental model systems and methodologies. Here we demonstrate the utility of optical interrogation of sympathetic neurons and their effects on macroscopic cardiac monolayer dynamics to address research targets such as the effects of adrenergic stimulation via the release of neurotransmitters, the effect of neuronal numbers on cardiac wave behaviour and the applicability of optogenetics in mechanistic in vitro studies. We combine photo-uncaging or optogenetic neural stimulation with imaging of cardiac monolayers to measure electrical activity in an automated fashion, illustrating the power and high throughput capability of such interrogations. The methods described highlight the challenges and benefits of co-cultures as experimental model systems. RESULTS:1] Stellate Sympathetic Neurons make contacts with cardiomyocytes: Scanning electron microscopy of sympathetic neurons growing in co-culture with cardiomyocytes in vitro shows connections between neurite extension and cardiac syncytium (Figure 1 A). The neuron bodies and extensions clearly make physical contact with myocytes (Figure 1 B, C, D, E, F, G, H, I, J). Immuno staining of co-cultures shows co-localisation of Synpasin and Beta2 receptors (Figure 1 K), sympathetic neurons show positive staining for Th (Figure 1 L), and fibronectin contamination in the co-cultures was assessed by staining with Vimentin (Figure 1 M), which showed low abundance. 2] Pattern formation is affected by the presence of neurons: Using dye-free imaging (Figure 2 A, B, C), we investigated how neuronal activation modulates cardiac patterns of activation in monolayer culture. We chose experimental conditions that spontaneously yield a wide variety of wavefront topologies within the imaging system's 16 x 16 mm field of view. Introduction of an additional cell type can potentially introduce heterogeneities that would impact wave front stability. Surprisingly, co-cultures displayed fewer wave breaks than their monoculture counterparts at similar plating densities (Figure 2 D). We broadly classified wave dynamics as simple (periodic target waves and single spiral wave reentry, Figure 2 D top row), or complex (single dominant spirals with additional irregular waves and multiple equally sized wavelets, Figure 2 D bottom row). While monocultures frequently displayed complex dynamics, co-cultures rarely displayed this behavior (Figure 2 E, P<0.05, chi-square test). Indeed, we observed wavelet reentry in only one of the co-culture preparations (6 isolations, 20 preparations). 3] Conduction velocity in spontaneously active co-cultures is faster than myocyte monocultures:We measured conduction velocity in unstimulated co-cultures (n=19) and myocyte monocultures (n=9). Figure 2 F s...

A ‘comb’ algorithm for accurate detection of calcium transients and action potentials in regularly activating cardiac preparations

“…Both linear and nonlinear drift are processed correctly by the comb algorithm ( Figure 3A,B). In addition, we tested the performance on very abrupt step-like change in the signal baseline; this arises during optogenetic experiments, when the preparation is illuminated by stepwise pulses [2,3]. The algorithm functions well even when the step boundary falls within a calcium transient ( Figure 3C).…”

“…For clarity, we assume the recording is taken at 1 KHz equidistant sampling. The algorithm can be nevertheless naturally extended for any sampling rate.2 It is not necessary to go beyond bcl ms, given that the resulting comb would be identical to a previously considered one 3. In our experience, 5 ms is sufficient for generally regular recordings, while 15 ms is enough even for recordings with marked beat-to-beat variability.…”