(448b) Vessel Wall Transport: Can Increasing the Endothelial Cell Expression of the Membrane Protein Aquaporin-1 Slow Down Pre-Lesion Atherosclerosis?
Macromolecular (e.g., low-density lipoprotein (LDL) cholesterol) transport across the arterial wall appears to be the earliest pre-atherosclerotic triggering event. This transport occurs mainly by advection: Transmural (i.e., across the vessel wall) pressure differences (i.e., blood pressure) drive plasma (water and small solutes) across the monolayer of endothelial cells (EC) that lines the inside of the artery wall. One EC every few thousand has junctions that are temporarily widened, some because they are either dying, dividing, or have stigmata, and the transmural flow can advect large tracers through them into the wall. A wall parameter, the hydraulic conductivity (Lp), defined as the ratio of the transmural water flux to hydrostatic pressure difference, is central to the understanding of this transmural water transport. On one hand, it advects macromolecules like LDL into the arterial subendothelial intima (SI) through these rare, widened EC leaks and spreads it in the SI, thereby giving it the chance to bind to extracellular matrix in the SI layer and possibly to trigger the start of atherosclerotic lesion formation. On the other hand, the overall water flow across the normal (non-leaky) endothelium dilutes the local SI LDL concentration, thereby likely slowing binding kinetics, and washing not-yet-bound lipid further into the wall. Our group’s discovery of the presence of the ubiquitous water channel membrane protein Aquaporin-1 (AQP) in rat aortic endothelial cells suggests a new possibility of water transport across the normal (non-leaky) endothelium, alongside the generally accepted paracellular route. Interestingly, we found that chemically blocking AQPs or knocking them down with siRNA changes rat aortic wall Lp in a strongly pressure dependent manner. We then proposed a new theory that agrees well with this otherwise perplexing experimentally-observed pressure-dependence. One upshot of this theory is that it suggests that AQPs contributes about 30% to the phenomenological endothelial Lp at low transmural pressures.
In the present work we investigate the effect of AQPs on the overall transport of various species, like Horseradish Peroxidase (HRP) and LDL, across the vessel endothelium and its further spread in the vessel wall using the advection-diffusion model developed by Huang et al. (Journal of Biomechanical Engineering, vol 116, 1994, pp. 430-445) and later improved by Zeng et al. (American Journal of Physiology, Heart Circ Physiol, 302, 2012, pp. 1683-1699). We extend this model by incorporating the trancellular water flow through AQPs and use some important measurable phenomenological parameters like the hydraulic conductivity of the endothelium plus intima (Lpe+i). We also include the intima thicknesses corresponding to a given Lpe+i. Our local filtration model mentioned above allows us to calculate how these important parameters vary with pressure, something that was not considered in earlier models. The goal of the present model is to investigate if increasing endothelial AQP expression – say, pharmacologically or via diet, which leads to higher intrinsic endothelial Lpe and which reduces SI compression causing higher overall vessel wall Lpt - can decrease the overall concentration of macromolecules in the SI and thereby slow down kinetics of its binding to SI ECM. This could have clinical relevance since such binding is believed to be the triggering event in lesion formation and subsequent pre-atherosclerotic events. Our theoretical findings predict that increasing transcellular transport by increasing AQP expression shifts the pressure regime over which Lpe+i is a strong function of pressure to higher pressure values. Based on these predictions, experiments in our lab tested and confirmed these predictions. The theory further predicts that such an upregulation lowers the concentration of HRP in the sub-endothelial space and similarly affects the more biologically-relevant macromolecule LDL. By November, we hope to have data on HRP transport into vessel walls with upregulated AQPs to evaluate our predictions.