High density lipoprotein (HDL), the carrier of so-called "good" cholesterol, serves as the major athero-protective lipoprotein and has emerged as a key therapeutic target for cardiovascular disease. We applied small angle neutron scattering (SANS) with contrast variation and selective isotopic deuteration to the study of nascent HDL to obtain the low resolution structure in solution of the overall time-averaged conformation of apolipoprotein AI (apoA-I) versus the lipid (acyl chain) core of the particle. Remarkably, apoA-I is observed to possess an open helical shape that wraps around a central ellipsoidal lipid phase. Using the low resolution SANS shapes of the protein and lipid core as scaffolding, an all-atom computational model for the protein and lipid components of nascent HDL was developed by integrating complementary structural data from hydrogen/deuterium exchange mass spectrometry and previously published constraints from multiple biophysical techniques. Both SANS data and the new computational model, the double superhelix model, suggest an unexpected structural arrangement of protein and lipids of nascent HDL, an anti-parallel double superhelix wrapped around an ellipsoidal lipid phase. The protein and lipid organization in nascent HDL envisages a potential generalized mechanism for lipoprotein biogenesis and remodeling, biological processes critical to sterol and lipid transport, organismal energy metabolism, and innate immunity. High density lipoprotein (HDL)2 functions in removal of cholesterol from peripheral tissues, such as within the artery wall, for delivery to the liver and ultimate excretion as biliary cholesterol within the intestinal lumen, a process called reverse cholesterol transport (1, 2). Plasma levels of HDL cholesterol and apolipoprotein AI (apoA-I), the major protein component of HDL, are inversely related to the risk of developing coronary artery disease (3-5). Moreover, genetic alterations that induce changes in apoA-I levels in both animals and humans alter susceptibility for development of atherosclerotic heart disease (3-6). Thus, numerous interventions aimed at enhancing reverse cholesterol transport are being examined as potential novel therapeutic interventions for the prevention and treatment of cardiovascular disease (7,8). Examples include methods for generating new HDL particles through enhanced production or delivery of either intact apoA-I (9, 10) or peptide mimetics of apoA-I (11), as well as modulating interactions between nascent HDL and proteins involved in HDL particle maturation and remodeling for potential therapeutic benefit (12-14). Structural elucidation often serves as the "Rosetta Stone" for enhanced understanding of function. It is thus remarkable that despite its importance to numerous biological and biomedical functions and its current prominent role as a target for therapeutic interventions, to date, the structures of neither the protein nor lipid components of nascent HDL have been directly visualized, and the high resolution structure of the particl...

High density lipoprotein (HDL) is the major atheroprotective particle in plasma. Recent studies demonstrate that myeloperoxidase (MPO) binds to HDL in vivo, selectively targeting apolipoprotein A1 (apoA1) of HDL for oxidative modification and concurrent loss in cholesterol efflux and lecithin cholesterol acyl transferase activating activities, generating a "dysfunctional HDL" particle. We now show that (patho)physiologically relevant levels of MPO-catalyzed oxidation result in loss of noncholesterol efflux activities of HDL including anti-apoptotic and anti-inflammatory functions. One mechanism responsible is shown to involve the loss of modified HDL binding to the HDL receptor, scavenger receptor B1, and concurrent acquisition of saturable and specific binding to a novel unknown receptor independent of scavenger receptors CD36 and SR-A1. HDL modification by MPO is further shown to confer pro-inflammatory gain of function activities as monitored by NF-B activation and surface vascular cell adhesion molecule levels on aortic endothelial cells exposed to MPO-oxidized HDL. The loss of non-cholesterol efflux activities and the gain of pro-inflammatory functions requires modification of the entire particle and can be recapitulated by oxidation of reconstituted HDL particles comprised of apoA1 and nonoxidizable phosphatidylcholine species. Multiple site-directed mutagenesis studies of apoA1 suggest that the pro-inflammatory activity of MPO-modified HDL does not involve methionine, tyrosine, or tryptophan, oxidant-sensitive residues previously mapped as sites of apoA1 oxidation within human atheroma. Thus, MPO-catalyzed oxidation of HDL results not only in the loss of classic atheroprotective reverse cholesterol transport activities of the lipoprotein but also both the loss of non-cholesterol efflux related activities and the gain of pro-inflammatory functions. High density lipoprotein (HDL)3 is a complex mixture of cholesterol carrying lipoprotein particles built upon a predominantly apolipoprotein A1 (apoA1) backbone. HDL is currently thought to function primarily in mediating reverse cholesterol transport (RCT), the net transport of cholesterol from peripheral tissues to the liver for ultimate elimination into the intestinal lumen as biliary cholesterol for excretion in feces (1). RCT involves multiple biochemical processes, including both lipid poor apoA1 and HDL serving as acceptors of cholesterol efflux from peripheral cholesterol loaded cells, maturation of HDL from a nascent relatively cholesterol poor particle into a cholesterol-laden spherical form through interaction with lecithin cholesterol acyl transferase (LCAT), and delivery of cholesterol to liver and steroidogenic tissues through the HDL receptor, scavenger receptor B1 (SR-B1) (2).Although the RCT related functions of apoA1 and HDL are thought to primarily account for both the atheroprotective activity and the strong inverse association of HDL cholesterol and apoA1 levels and cardiovascular risks, other non-cholesterol efflux-related activities have a...

Spherical high density lipoprotein (sHDL), a key player in reverse cholesterol transport and the most abundant form of HDL, is associated with cardiovascular diseases. Small angle neutron scattering with contrast variation was used to determine the solution structure of protein and lipid components of reconstituted sHDL. Apolipoprotein A1, the major protein of sHDL, forms a hollow structure that cradles a central compact lipid core. Three apoA1 chains are arranged within the low resolution structure of the protein component as one of three possible global architectures: (i) a helical dimer with a hairpin (HdHp), (ii) three hairpins (3Hp), or (iii) an integrated trimer (iT) in which the three apoA1 monomers mutually associate over a portion of the sHDL surface. Cross-linking and mass spectrometry analyses help to discriminate among the three molecular models and are most consistent with the HdHp overall architecture of apoA1 within sHDL.Epidemiological studies firmly establish that circulating levels of high density lipoprotein (HDL) cholesterol and apolipoprotein A1 (apoA1), 2 the major protein constituent of HDL particles, are inversely associated with atherosclerotic heart disease risk (1-3). Moreover, genetic studies further confirm a strong mechanistic link between HDL and apoA1 and cardiovascular disease (4 -8). Defined by its buoyant density characteristics, HDL represent a heterogeneous group of particles with varied lipid composition and protein content that participate in diverse biological functions ranging from lipid transport to innate immune functions. For example, HDL serves as an acceptor of cholesterol from peripheral tissue macrophages and promotes lipid transport through delivery of cholesterol to the liver and steroidogenic tissues (9 -11). HDL also mediates systemic anti-inflammatory and anti-oxidant functions (12-14), and HDL-associated proteins can play critical host defense functions (15). ApoA1 represents nearly three-quarters of the protein content of HDL by mass, and it plays a central functional role in facilitating the numerous biological activities of HDL. Typically present at 2-4 molecules/particle depending upon the degree of HDL maturation, apoA1 serves as the fundamental structural element of the particle (16, 17) and is critical for specific interactions with proteins involved in HDL biogenesis (18, 19), maturation and remodeling (20,21), and recognition by target organ receptors (22,23).Because HDL can be generated in a relatively homogenous form, most structural studies of HDL have focused on reconstituted nascent HDL, a particle composed of two molecules of apoA1 associated with phospholipid and free cholesterol (16,17). Early small angle neutron scattering (SANS) and small angle x-ray scattering (SAXS) studies of nascent HDL particles were reported nearly 3 decades ago and are consistent with an outer protein layer relative to a central lipid core (24,25). Current structural models of nascent HDL have an anti-parallel apoA1 chain orientation and posit that the protein exists...

The most frequent injury sustained by US service members deployed to Iraq or Afghanistan is mild traumatic brain injuries (mTBI), or concussion, by far most often caused by blast waves from improvised explosive devices or other explosive ordnance. TBI from all causes gives rise to chronic neuroendocrine disorders with an estimated prevalence of 25–50%. The current study expands upon our earlier finding that chronic pituitary gland dysfunction occurs with a similarly high frequency after blast-related concussions. We measured circulating hormone levels and accessed demographic and testing data from two groups of male veterans with hazardous duty experience in Iraq or Afghanistan. Veterans in the mTBI group had experienced one or more blast-related concussion. Members of the deployment control (DC) group encountered similar deployment conditions but had no history of blast-related mTBI. 12 of 39 (31%) of the mTBI participants and 3 of 20 (15%) veterans in the DC group screened positive for one or more neuroendocrine disorders. Positive screens for growth hormone deficiency occurred most often. Analysis of responses on self-report questionnaires revealed main effects of both mTBI and hypopituitarism on postconcussive and posttraumatic stress disorder (PTSD) symptoms. Symptoms associated with pituitary dysfunction overlap considerably with those of PTSD. They include cognitive deficiencies, mood and anxiety disorders, sleep problems, diminished quality of life, deleterious changes in metabolism and body composition, and increased cardiovascular mortality. When such symptoms are due to hypopituitarism, they may be alleviated by hormone replacement. These findings suggest consideration of routine post-deployment neuroendocrine screening of service members and veterans who have experienced blast-related mTBI and are reporting postconcussive symptoms.