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Title: The influence of nicotinic acid adenine dinucleotide phosphate (NAADP) signalling and the significance of the organization of lysosomes in cardiac myocytes
Author: Aston, Daniel
ISNI:       0000 0004 6499 1923
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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The central role of calcium ions in the generation of contractile force in the heart has been understood for many decades. In many mammals, a large proportion of the calcium responsible for activation of the contractile apparatus is released from the sarcoplasmic reticulum (SR), and this release is regulated via many mechanisms. One regulatory factor that has earned much attention in recent years is Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP) and its probable receptor, Two-Pore Channel-2 (TPC2), which is found on the surface of lysosomes. NAADP has been found to function in a wide range of tissues from both animals and plants. In the heart, it is thought that NAADP may be generated in response to stimulation of cardiomyocytes with β-adrenergic agonists and that it acts at the lysosome to cause release of calcium. This calcium may then regulate the release and reuptake of calcium by the SR. Due to the importance of calcium in the heart, substances that modulate calcium release in cardiac cells may be expected to have a broad array of effects. Under some circumstances, the dysregulation of calcium signalling caused by these substances may also be expected to have pathological consequences, both acutely and over the long term. The exposure of ex-vivo hearts perfused via Langendorff apparatus to the -adrenergic agonist isoprenaline resulted in ventricular arrhythmias following electrical burst pacing in wildtypes, but not in hearts from TPC2 knockout (Tpcn2-/-) animals. This data supports the hypothesis that NAADP is important in the acute cardiac response to -agonists, and more specifically that it has an influence on the calcium handling events that lead to arrhythmogenesis in these cells. Similarly, chronic treatment of cultured neonatal rat cardiomyocytes with the adrenergic agonists isoprenaline and noradrenaline is known to induce a hypertrophic response associated with re-activation of the foetal gene program. This response was found to be strongly attenuated by the NAADP antagonist Ned-19, which prevented both the increase in cell size and the increase in expression of foetal genes associated with hypertrophy. These results suggest that in addition to the role of NAADP in acute responses to β-adrenergic agonists, it may also be important in chronic responses. Calcium released from the lysosome in response to NAADP action at TPC2 is thought to modulate calcium release from the SR. This process is likely to require a close spatial relationship between these organelles. Data from confocal microscopy experiments demonstrate an association between lysosomes and the SR, while membrane contact sites (MCS) between these structures are shown to exist using electron microscopy. These observations support the hypothesis that functional calcium signalling microdomains exist between lysosomes and the SR in the heart.
Supervisor: Terrar, Derek Sponsor: British Heart Foundation
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available