Biophysics of naked mole-rat derived lipid bilayers and their complex behaviour

Abstract

The naked mole-rat (NMR) is an exceptionally long lived rodent, living significantly longer than other similarly sized rodents. In this thesis the lipid component of the NMR cell membrane was assessed to determine the role it plays in this longevity at the molecular/membrane level. Atomic force microscopy reveals that supported lipid bilayers formed from NMR lipids exhibit a high degree of phase separation compared with mouse lipids. This phase behaviour is unique when compared with commercially available lipid extracts derived from other animals. Such phase separation does not provide protection from the damage induced by exposure to amyloid beta, a key player thought to be involved in Alzheimer’s disease. Compared with mouse derived supported lipid bilayers there is increased damage. Key compositional differences between the NMR and mouse lipids were identified in cholesterol and sphingomyelin, both of which are linked with lipid bilayer phase separation. Sphingomyelin was focused on further due to its relation with Alzheimer’s disease, phase separation and the difference in tail length distributions between the two animals, with NMR having much shorter chain lipids, whereas mouse had more highly asymmetric, longer chain lipids. At the temperature studied, sphingomyelin, at any chain length/asymmetry, was found in the ripple phase, rather than the commonly studied fluid phase. At large tail asymmetries, there was a high degree of variability in the lipid tail conformations, with the tails bending to accommodate the unbalanced lipid shapes. A new technique for quantifying the ripple phase in molecular dynamics simulations was developed utilising a three-body correlation function to measure local structure and unsupervised machine learning. This was able to identify four unique lipid conformations in the ripple phase of the widely simulated DPPC lipid bilayer. This technique has the potential to be applied to more complex systems, like the NMR composition lipids