Abstracts

Rationale: Dravet Syndrome (DS) is a severe pediatric epileptic encephalopathy. Patients with DS have an increased risk of Sudden Unexpected Death in Epilepsy (SUDEP). One factor believed to contribute to SUDEP is cardiac arrhythmias. Mouse models of DS with mutations in the genes Scn1a and Scn1b show an increased incidence of arrhythmias, alongside altered Na⁺ and Ca²⁺ handling in isolated cardiomyocytes. In the heart, Ca²⁺ are sequestered by the mitochondria where they play a role in matching ATP supply with demand. Under conditions of elevated intracellular Na+, this process can be compromised and affect cardiac function. Finally, we have previously shown that mitochondrial respiration, the process by which the heart generates the large majority of its ATP, is compromised in multiple DS models. We hypothesize that hearts from the Scn1a^-/+ and Scn1b^-/- DS mouse models have altered mitochondrial Ca²⁺ sequestration, predisposing them to cardiac arrhythmias.


Methods: First, we isolated mitochondria from DS and wild-type ventricular samples. Next, a high-resolution respirometry was used in conjunction with the extramitochondrial Ca²⁺ indicator, CaGreen, to assess rates of mitochondrial Ca²⁺ uptake and retention (the maximum level of Ca²⁺ mitochondria could buffer). During recordings, we evaluated O₂ consumption in energized mitochondria to assess the ability of Ca²⁺ to impact respiration rate. Lastly, we evaluated the expression of proteins associated with mitochondrial Ca²⁺ uptake.


Results: We found that in the Scn1a^-/+ model, Ca²⁺ uptake and retention were unchanged from Scn1a^+/+ cardiac mitochondria (p = 0.94 - 0.98). There was also no effect on respiration in this model (p = 0.27). Conversely, in Scn1b^-/- female hearts, mitochondrial Ca²⁺ uptake was increased compared to Scn1b^+/+ (p = 0.02) despite no change in Ca²⁺ retention values (p = 0.91). Also, the ability of Ca²⁺ to stimulate mitochondrial respiration was significantly attenuated in mitochondria from Scn1b^-/- hearts (p = 0.0005). Scn1b^-/- cardiac samples additionally showed imbalances in gene expression of proteins involved in mitochondrial Ca²⁺ uptake.


Conclusions: Overall, these results indicate that, especially in the Scn1b model of DS, dysfunction in mitochondrial sequestration of Ca²⁺ may contribute to altered cardiomyocyte Ca²⁺ dynamics and increased arrhythmia susceptibility.

Funding: This work was supported by the National Institute of Neurological Disorders and Stroke [R21NS116647] at the National Institutes of Health; and the Congressionally Directed Medical Research Programs [EP220018]