Calcium signalling in atrial myocytes and potential targets for atrial fibrillation

Thesis

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia (incidence 1-2%). Age-related increases in the risk for AF contribute to 14% of strokes in the UK, which is a significant health burden. Certain treatment modalities, like catheter ablation, have some associated morbidity/mortality concerns and are not always 100% effective. Traditional antiarrhythmic drugs do not have specific intracellular targets; hence it is critical to comprehend the molecular pathophysiology of AF. Calcium homeostasis is a vital signalling mechanism in the heart and alterations in the intracellular concentration of calcium have been shown to lead to arrhythmias. My thesis involves investigations of calcium handling pathways through adrenoceptor stimulation, using isoprenaline for β-adrenergic stimulation and phenylephrine for α-adrenergic stimulation.

I investigated the very potent lysosomal calcium releasing molecule nicotinic acid adenine dinucleotide phosphate (NAADP) and I present evidence that lysosomal calcium, acting through NAADP-mediated stimulation of two-pore channels 2 (TPC2), makes a significant contribution to atrial function and pacemaking. My experiments reveal that lysosomal calcium release directly modulates cyclic adenosine monophosphate (cAMP) upstream to the previously reported effects on sarcoplasmic reticulum (SR) calcium content and calcium handling in neonatal rat atrial myocytes and human induced pluripotent stem cell derived cardiomyocytes. Electron microscopy measurements highlight a close spatial relationship between lysosomes and the SR to be altered in human AF tissue samples. In addition to NAADP signalling, I investigated inositol 1,4,5-trisphosphate (IP3) signalling. The work presented here brings together two major aspects of calcium handling: NAADP signalling and IP3 signalling and for the first-time probes these pathways in human cells. The experiments investigate the interplay between IP3-mediated calcium release from the SR and NAADP-mediated calcium release from lysosomes in atrial cells.

In conclusion, my thesis provides evidence that IP3 and NAADP regulate the release of calcium from intracellular stores in atrial myocytes and influence cardiac function including pacemaking and presents the translational value of my research by investigating these pathways in humans.

Authors: Akerman, ECL

• DOI: 10.5287/ora-eoy7ze1j7
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: cyclic adenosine monophosphate; calcium; nicotinic acid adenine dinucleotide phosphate; lysosome; atrial fibrillation
• Subjects: Calcium signalling