Understanding the mechanisms of phosphatidylserine exposure in sickle cells

Abstract

Sickle cell disease (SCD) is the most common severe inherited disorder affecting millions of people worldwide. HbS polymerisation leads to a change in red blood cells’ (RBCs) membrane permeability, high phosphatidylserine (PS) exposure, altered RBC rheology and fragility. The high PS exposure is considered to cause some of the hallmark complications of the disease such as vascular occlusion, anaemia and inflammation. This study investigates the possible physiological and cellular signalling pathways involved in PS exposure in RBCs from SCD patients. In the first part, conditions specific to the renal medulla were examined as the majority of SCD patients suffer from nephropathy early in life. It is hypothesised that the ambient conditions of the renal medulla promote polymerisation of RBCs and PS exposure, and so contribute to the detrimental effects of SCD. Thus, the impact of the medullary environment, which is hypoxic, acidotic, hyperosmotic and hypertonic, on RBCs of SCD patients was investigated. In the second part of the thesis, intracellular signalling pathways which could cause high PS exposure were investigated. The study aimed to establish the molecular identity of Psickle and strengthen the link between Psickle and PS exposure. It was hypothesized that PIEZO1, a mechanosensitive ion channel, is a potential candidate for Psickle and a major channel for Ca2+ entry in RBCs. Thus, drugs acting on PIEZO1 such as Yoda1, Dooku1 and GsMTx4 were used to examine their effects on Ca2+ entry and PS exposure. The results from the first part of the thesis suggest that while the hypoxic, hyperosmotic and hypertonic environment have a substantial effect on sickling and PS exposure, the effect of pH is minimal. Furthermore, the experiments have also shown that urea inhibits both sickling and PS exposure (highly significantly in all the above conditions). Results from the second part of the thesis strongly suggest that PIEZO1 can be a major channel for Ca2+ entry leading to PS exposure, together with a Ca2+-independent pathway leading to PS exposure, which is reliant on protein kinase C (PKC). Furthermore, the sphingomyelinase (SMase) signalling pathway was also explored to identify its role in PS exposure. Results with a SMase inhibitor and ceramide on RBCs of SCD patients showed a strong correlation between SMase activity and PS exposure. Moreover, experiments revealed that urea inhibited SMase strongly and might be potentially working through this pathway to reduce PS exposure. These findings further increase our understanding of the conditions and mechanisms, which promote PS exposure and suggest potential future therapeutic targets.