Ph.D Thesis Colloquium : Anant Verma

Effect of oxysterols on G-protein coupled receptor (GPCR) signalling and phospholipid membrane properties

When cells are exposed to various levels of oxidative stress, membrane phospholipids, sterols and cellular proteins undergo a range of oxidative reactions. Cholesterol oxidation results in formation of oxysterols which alters physicochemical, mechanical and protein mediated signalling events. The first part of the thesis focuses on the mitigating effect of oxysterols on signalling by G-protein coupled receptors (GPCRs) implicated in ageing and senescence. Chemokine receptor type 4 (CXCR4), a GPCR which mediates signalling for cell proliferation and migration requires cholesterol for normal functioning. All-atom molecular dynamics simulations of CXCR4 in membranes with different oxysterol compositions reveal that the tail-oxidized sterol, 27-hydroxysterol, displaces cholesterol and binds to several critical signalling residues. These trends were however significantly reduced for the ring-oxidized sterols providing molecular insights into reduced calcium signalling with tail-oxidized sterols reported in experiments. This is the first study of its kind where molecular aspects of oxysterol-protein interactions are analysed and related to the influence of GPCR signalling pathways. The second part of the thesis focuses on electroporation simulations of lipid membranes containing oxysterols – relevant to electroporation of aged and tumour cells. We show that the pore-forming process in the presence of tail-oxidized sterols is drastically different from membranes with cholesterol or ring-oxidized sterols. The presence of an extra OH group at the centre of the bilayer in tail-oxidized sterols is seen to enhance the pore-forming by forming hydrogen bonds with water to provide a hydrophilic pathway for water molecules to form membrane spanning bridges.