Pulmonary arterial hypertension (PAH) is a rare but devastating disease characterised by the progressive remodelling of the small pulmonary vessels. Although the causes may vary, similar pathobiological features are shared among different forms of PAH, with endothelial dysfunction, the hyperproliferation of smooth muscle cells and mesenchymal cells in the vascular wall, as well as inflammation contributing to this process. Despite the availability of licensed therapies, 5-year survival for patients remains less than 60%. To uncover additional fundamental mechanisms of PAH pathobiology, our group undertook whole genome sequencing in 1038 idiopathic or heritable PAH patients and identified disease-associated heterozygous mutations in ATP13A3, a P5B-type ATPase. P5 ATPases, namely ATP13A1-5, are putative transmembrane proteins that generate and maintain significant chemical gradients across biological membranes by active transport of cations. However, to date, studies of their functions are sparse. Therefore, the focus of my project was to characterise ATP13A3 in the pulmonary vasculature and to uncover the underlying mechanism of how mutations in this gene contribute to PAH pathogenesis. Although the biological functions of ATP13A3 are elusive, its homologue (CATP-5) in C. elegans was previously reported as a polyamine transporter. Polyamines are a group of naturally existing polycations essential for many cellular processes. Therefore, my working hypothesis was that ATP13A3 loss of function might disrupt the polyamine homeostasis and hence contribute to the abnormal vascular remodelling in PAH. To address this, I first confirmed ATP13A3 as residing in the recycling endosome of endothelial cells. This localisation of ATP13A3 suggested it might be related to polyamine transport since the endocytic pathway was recognised as a crucial route for polyamine trafficking. To expand this finding, I developed molecular tools to knockdown or to overexpress ATP13A3 in vascular cells and analysed how these affected polyamine transport. This led to the discovery of putrescine as the preferred substrate for ATP13A3 and also revealed a basal reduction of other polyamines in endothelial cells. Further investigation highlighted an essential role of ATP13A3 in pulmonary vasculature with its deficiency restrained cell growth and predisposed endothelial cells to apoptosis. To validate these findings, blood outgrowth endothelial cells (BOECs) were isolated from a PAH patient bearing an LK726X ATP13A3 frameshift mutation (LK726X). Intriguingly, ATP13A3 causing ATP13A3 reduction, impaired polyamine uptake and induced apoptosis in BOECs. This provides the first evidence for the pathogenic effects of a PAH associated ATP13A3 mutation. In addition, using a lentiviral overexpression system, I also demonstrated impairments of other PAH- associated mutations on ATP13A3 mediated polyamine uptake, reinforcing the involvement of ATP13A3 in PAH pathobiology. To explore whether ATP13A3 mutations can cause PAH, our laboratory has generated a mouse model harbouring a human ATP13A3 mutation (P452Lfs) via the MRC Harwell Gene editing service. My colleague, Dr Ekaterina Legchenko and I have discovered that the Atp13a3P452Lfs/P452Lfs mice spontaneously develop PAH-like hemodynamic changes at 6-months of age. Preliminary data also revealed lower Atp13a3 level and reduced polyamine content in the lungs of the mutant mice, suggesting that Atp13a3 frameshift mutation (P452L) may contribute to PAH pathogenesis via the disruption of polyamine homeostasis in vivo. In conclusion, my work characterises for the first time that ATP13A3 is a polyamine transporter in vascular cells with putrescine as its preferable transport substrate. ATP13A3 mutations identified from PAH patients are highly likely to impair polyamine homeostasis and contribute to PAH progression. These findings open up a new field for investigating PAH pathobiology and also highlight the potential role of rebalancing polyamine homeostasis in PAH treatment.