Membrane properties of hydroxycholesterols related to the brain cholesterol metabolism

Membrane organization and intracellular transport of a fluorescent analogue of 27-hydroxycholesterol

Oxysterols are cholesterol metabolites with multiple functions in controlling cellular homeostasis. In particular, 27-hydroxycholesterol (27-OH-Chol) has been shown to regulate a variety of physiological functions, but little is known about its uptake, intracellular trafficking, and efflux from cells. This is largely due to a lack of suitable analogs of 27-OH-Chol, which mimic this oxysterol closely. Here, we present the intrinsically fluorescent 27-hydroxy-cholestatrienol (27-OH-CTL), which differs from 27-OH-Chol only by having two additional double bonds in the steroid ring system. Based on molecular dynamics (MD) simulations, we show that 27-OH-CTL possesses almost identical membrane properties compared to 27-OH-Chol. By comparative imaging of 27-OH-CTL and of the cholesterol analogue cholestatrienol (CTL) in living cells, we assess the impact of a single hydroxy group on sterol trafficking. We find that human fibroblasts take up more CTL than 27-OH-CTL, but efflux the oxysterol analogue more efficiently. For both sterols, efflux includes shedding of vesicles from the plasma membrane. Intracellular, 27-OH-CTL accumulates primarily in lipid droplets (LDs), while CTL is mostly found in endosomes and lysosomes. Using fluorescence recovery after photobleaching (FRAP), we find for both sterols a rapidly exchanging pool, which moves orders of magnitude faster than sterol containing vesicles and LDs. In summary, by applying a new fluorescent derivative of 27-OH-Chol we demonstrate that human cells can distinguish sterols based on a single hydroxy group in the side chain, resulting in different transport itineraries, dynamics, and efflux kinetics. Both intrinsically fluorescent cholesterol and oxysterol analogues show rapid non-vesicular transport in human fibroblasts.

Frontiers of Complex Disease Mechanisms: Membrane Surface Tension May Link Genotype to Phenotype in Glaucoma

Although many monogenic diseases are understood based upon structural changes of gene products, less progress has been made concerning polygenic disease mechanisms. This article presents a new interdisciplinary approach to understand complex diseases, especially their genetic polymorphisms. I focus upon primary open angle glaucoma (POAG). Although elevated intraocular pressure (IOP) and oxidative stress are glaucoma hallmarks, the linkages between these factors and cell death are obscure. Reactive oxygen species (ROS) promote the formation of oxidatively truncated phosphoglycerides (OTP), free fatty acids, lysophosphoglycerides, oxysterols, and other chemical species that promote membrane disruption and decrease membrane surface tension. Several POAG-linked gene polymorphisms identify proteins that manage damaged lipids and/or influence membrane surface tension. POAG-related genes expected to participate in these processes include: ELOVL5, ABCA1, APOE4, GST, CYP46A1, MYOC, and CAV. POAG-related gene products are expected to influence membrane surface tension, strength, and repair. I propose that heightened IOP overcomes retinal ganglion cell (RGC) membrane compressive strength, weakened by damaged lipid accumulation, to form pores. The ensuing structural failure promotes apoptosis and blindness. The linkage between glaucoma genotype and phenotype is mediated by physical events. Force balancing between the IOP and compressive strength regulates pore nucleation; force balancing between pore line tension and membrane surface tension regulates pore growth. Similar events may contribute to traumatic brain injury, Alzheimer's disease, and macular degeneration.