ESR 2 - Tipparat Parakaw

XOR: the missing link in platelet hyperactivity and cardiovascular disease?

When we think about platelets, it is well known that they help our body form clots at sites of vascular injury and maintain the integrity of the vasculature. Without platelets, we would have severe bleeding which would cause death. Less is known, however, on how platelet hyperactivation can contribute to pathologic thrombosis and other diseases, including ischemic stroke, chronic atherosclerosis, myocardial infarction (MI), congestive heart failure (CHF), diabetes mellitus, renal disease and pulmonary arterial hypertension (PAH) (Sangkuhl et al, PMID: 20938371). The most recent cohort study this year with 2831 participants revealed evidence that individuals who were hyperresponsive to Adenosine diphosphate (ADP), a platelet stimulant, showed higher risk of stroke, MI, and cardiovascular death (Puurunen et al. PMID: 29502103). Therefore, finding an approach to inhibit platelet hyperactivity is potentially one of the most pivotal strategies to reduce the risk of cardiovascular diseases.

Interestingly, inorganic nitrate rich in green leafy vegetables (especially spinach and beetroot) has been shown to exert a substantial benefit upon platelet regulation (Webb et al. PMID: 18250365, Velmurugan et al. PMID: 26607938). The benefits on cardiovascular function have been attributed to the bioconversion of inorganic nitrate to nitrite and then from nitrite to nitric oxide (NO) which decreases platelet aggregation and blood pressure (Figure 1). The first step in this chemical pathway is the conversion of nitrate (NO3-) to nitrite (NO2-) by commensal bacteria in the oral cavity. The second part of this pathway is a mammalian response. Several distinct molecular pathways have been shown to facilitate the reduction of nitrite to nitric oxide including heme-based nitrite reductases, molybdopterin reductases, and nitrite anhydrases. Xanthine oxidoreductase (XOR) is possibly the most important nitrite reductase enzyme which reduces nitrite to NO particularly in the settings of cardiovascular disease and endothelial dysfunction.

My PhD study aims to explore the functions of XOR in long-term dietary nitrate intake by using a genetic mouse model assessing cardiovascular function in wild type (control), Xdh heterozygous, and Xdh knockout mice, which is a gene coding for the XOR enzyme. My work will focus on assessing platelet function under cardiovascular disease scenarios where the conventional pathways for endogenous NO generation are impaired by using various techniques such as platelet aggregometery flow cytometry, intravital microscopy, tail bleeding, and chemiluminescence. This project will identify whether the enzyme XOR is key for anti-platelet activity in a cardiovascular disease scenario as well as determine whether targeting this pathway might be useful in therapeutics. I really hope that the exploration of this study will potentially bring to light a new drug target through the use of dietary nitrate supplementation as a therapeutic agent for the treatment of cardiovascular diseases, particularly platelet hyperactivation.