High-density lipoprotein (HDL) particles exhibit considerable heterogeneity in size, function, and composition, which traditional HDL measurement methods, typically based on HDL-cholesterol (HDL-C) levels, fail to capture. This limited approach constrains our understanding of HDL's response to lifestyle interventions and its therapeutic potential. This dissertation addresses these gaps through three chapters.
In Chapter 1, we examine the effects of lifestyle interventions, such as fasting, on HDL. Our findings indicate that the inconclusive results seen in previous studies stem from several factors: limitations in intervention standardization, the imprecision of HDL-C concentration measurement, and the lack of functional evaluation of HDL particles (HDL-P). Though limited, our findings reveal that small HDL-P levels decrease in the fasted state, with some studies showing an increase in HDL's cholesterol efflux capacity. These findings underscore the need for a more precise and functional analysis of HDL to better understand how lifestyle interventions impact it.
In Chapter 2, we explore ways to dissect HDL-C measurements into more detailed HDL-P measurements across subtypes, with a focus on evaluating their function. Using improved HDL subclass characterization, we investigate HDL’s potential as a therapeutic agent for reducing cholesterol accumulation in activated microglia—a hallmark of Alzheimer’s disease. Our study reveals that the HDL from a single bout of 36 hours of water-only fast effectively reduces cholesterol load in microglia treated with amyloid beta oligomers and cholesterol, a model that mimics Alzheimer’s pathogenesis. Furthermore, our results show that fasting remodels both the particle size distribution and proteomic composition of HDL, raising the question of whether isolating HDL by specific subclass and function could offer deeper insights into how other interventions affect HDL's properties and whether they possess characteristics that are telling of a person’s state of health.
Building on these findings, Chapter 3 presents a novel method for isolating, separating, and quantifying HDL particles by subclass, enabling more precise functional and compositional analysis. Our method demonstrates that HDL particles across subfractions retain their structure and function and are comparable to results from other lipoprotein analysis techniques. Additionally, we were able to characterized dense HDL particles, capturing their structure through electron microscopy (EM) and profiling their proteomic, lipidomic, and RNA content—marking the first time such detailed characterization has been completed.
Overall, this dissertation offers a more comprehensive approach to understanding HDL, its response to lifestyle interventions, and its potential therapeutic applications, advancing the field toward more precise and functional HDL measurements that could support targeted applications in precision health.
