Cholesterol is an essential component of both the peripheral nervous system and central nervous system (CNS) of mammals. Brain cholesterol is synthesized in situ by astrocytes and oligodendrocytes and is almost completely isolated from other pools of cholesterol in the body, but a small fraction can be taken up from the circulation as 27-hydroxycholesterol, or via the scavenger receptor class B type I. Glial cells synthesize native high-density lipoprotein (HDL)-like particles, which are remodelled by enzymes and lipid transfer proteins, presumably as it occurs in plasma. The major apolipoprotein constituent of HDL in the CNS is apolipoprotein E, which is produced by astrocytes and microglia. Apolipoprotein A-I, the major protein component of plasma HDL, is not synthesized in the CNS, but can enter and become a component of CNS lipoproteins. Low HDL-C levels have been shown to be associated with cognitive impairment and various neurodegenerative diseases. On the contrary, no clear association with brain disorders has been shown in genetic HDL defects, with the exception of Tangier disease. Mutations in a wide variety of lipid handling genes can result in human diseases, often with a neuronal phenotype caused by dysfunctional intracellular lipid trafficking.

A molecular understanding of high-density lipoprotein (HDL) will allow a more complete grasp of its interactions with key plasma remodelling factors and with cell-surface proteins that mediate HDL assembly and clearance. However, these particles are notoriously heterogeneous in terms of almost every physical, chemical and biological property. Furthermore, HDL particles have not lent themselves to high-resolution structural study through mainstream techniques like nuclear magnetic resonance and X-ray crystallography; investigators have therefore had to use a series of lower resolution methods to derive a general structural understanding of these enigmatic particles. This chapter reviews current knowledge of the composition, structure and heterogeneity of human plasma HDL. The multifaceted composition of the HDL proteome, the multiple major protein isoforms involving translational and posttranslational modifications, the rapidly expanding knowledge of the HDL lipidome, the highly complex world of HDL subclasses and putative models of HDL particle structure are extensively discussed. A brief history of structural studies of both plasma-derived and recombinant forms of HDL is presented with a focus on detailed structural models that have been derived from a range of techniques spanning mass spectrometry to molecular dynamics.

There are several potential mechanisms by which HDLs protect against the development of vascular disease. One relates to the unique ability of these lipoproteins to remove cholesterol from the arterial wall. Another is the ability of HDL to prevent and eventually correct endothelial dysfunction, a key variable in the pathogenesis of atherosclerosis and its complications. HDLs help maintain endothelial integrity, facilitate vascular relaxation, inhibit blood cell adhesion to vascular endothelium, reduce platelet aggregability and coagulation, and may favor fibrinolysis. These functions of HDLs complement their activity in arterial cholesterol removal by providing an excellent rationale for favorably influencing pathological processes underlying a variety of clinical conditions, such as accelerated atherosclerosis, acute coronary syndromes, and restenosis after coronary angioplasty, through a chronic or acute elevation of plasma HDL concentration.

Paraoxonase-1 (PON1) is a high density lipoprotein (HDL)-associated serum enzyme that protects low density lipoproteins from oxidative modifications. There is a relative lack of information on mechanisms implicated in PON1 release from cells. The present study focused on a model derived from stable transfection of CHO cells, to avoid co-secretion of apolipoprotein (apo) A-I and lipids, which could lead to formation of HDL-like complexes. Our results indicate that, in the absence of an appropriate acceptor, little PON1 is released. The results designate HDL as the predominant, physiological acceptor, whose efficiency is influenced by size and composition. Neither lipid-poor apoA-I or apoA-II nor low density lipoproteins could substitute for HDL. Protein-free phospholipid complexes promoted PON1 release. However, the presence of both apolipoprotein and phospholipid were necessary to promote release and stabilize the enzyme. Immunofluorescence studies demonstrated that PON1 was inserted into the external membrane of CHO cells, where it was enzymatically active. Accumulation of PON1 in the cell membrane was not influenced by the ability of the cell to co-secrete of apoA-I. Release appeared to involve desorption by HDL; human and reconstituted HDL promoted PON1 release in a saturable, high affinity manner (apparent affinity 1.59 ± 0.3 μg of HDL protein/ml). Studies with PON1-transfected hepatocytes (HuH-7) revealed comparable structural features with the peptide located in a punctate pattern at the external membrane and enzymatically active. We hypothesize that release of PON1 involves a docking process whereby HDL transiently associate with the cell membrane and remove the peptide from the external membrane. The secretory process may be of importance for assuring the correct lipoprotein destination of PON1 and thus its functional efficiency.

To better understand the role of LCAT in HDL metabolism, we compared HDL subpopulations in subjects with homozygous (n = 11) and heterozygous (n = 11) LCAT deficiency with controls (n = 22). Distribution and concentrations of apolipoprotein A-I (apoA-I)-, apoA-II-, apoA-IV-, apoC-I-, apoC-III-, and apoE-containing HDL subpopulations were assessed. Compared with controls, homozygotes and heterozygotes had lower LCAT masses (−77% and −13%), and LCAT activities (−99% and −39%), respectively. In homozygotes, the majority of apoA-I was found in small, disc-shaped, poorly lipidated preβ-1 and α-4 HDL particles, and some apoA-I was found in larger, lipid-poor, discoidal HDL particles with α-mobility. No apoC-I-containing HDL was noted, and all apoA-II and apoC-III was detected in lipid-poor, preβ-mobility particles. ApoE-containing particles were more disperse than normal. ApoA-IV-containing particles were normal. Heterozygotes had profiles similar to controls, except that apoC-III was found only in small HDL with preβ-mobility. Our data are consistent with the concepts that LCAT activity: 1) is essential for developing large, spherical, apoA-I-containing HDL and for the formation of normal-sized apoC-I and apoC-III HDL; and 2) has little affect on the conversion of preβ-1 into α-4 HDL, only slight effects on apoE HDL, and no effect on apoA-IV HDL particles.