Objective-Recent proteomic studies have identified multiple proteins that coisolate with human HDL. We hypothesized that distinct clusters of protein components may distinguish between physicochemically-defined subpopulations of HDL particles, and that such clusters may exert specific biological function(s). Methods and Results-We investigated the distribution of proteins across 5 physicochemically-defined particle subpopulations of normolipidemic human HDL (HDL2b, 2a, 3a, 3b, 3c) fractionated by isopycnic density gradient ultracentrifugation. Liquid chromatography/electrospray mass spectrometry identified a total of 28 distinct HDLassociated proteins. Using an abundance pattern analysis of peptide counts across the HDL subfractions, these proteins could be grouped into 5 distinct classes. A more in-depth correlational network analysis suggested the existence of distinct protein clusters, particularly in the dense HDL3 particles. Levels of specific HDL proteins, primarily apoL-I, PON1, and PON3, correlated with the potent capacity of HDL3 to protect LDL from oxidation. Conclusions-These findings suggest that HDL is composed of distinct particles containing unique (apolipo)protein complements. Such subspeciation forms a potential basis for understanding the numerous observed functions of HDL. Further work using additional separation techniques will be required to define these species in more detail.

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Recent genome-wide association studies have identified a genetic locus at human chromosome 8q24 as having minor alleles associated with lower levels of plasma triglyceride (TG) and LDL cholesterol (LDL-C), higher levels of HDL-C, as well as decreased risk for myocardial infarction. This locus contains only one annotated gene, tribbles homolog 1 (TRIB1), which has not previously been implicated in lipoprotein metabolism. Here we demonstrate a role for Trib1 as a regulator of lipoprotein metabolism in mice. Hepatic-specific overexpression of Trib1 reduced levels of plasma TG and cholesterol by reducing VLDL production; conversely, Trib1-knockout mice showed elevated levels of plasma TG and cholesterol due to increased VLDL production. Hepatic Trib1 expression was inversely associated with the expression of key lipogenic genes and measures of lipogenesis. Thus, we provide functional evidence for what we believe to be a novel gene regulating hepatic lipogenesis and VLDL production in mice that influences plasma lipids and risk for myocardial infarction in humans.

Adeno-associated viral (AAV) vectors are a leading candidate for the delivery of CRISPR-Cas9 for therapeutic genome editing in vivo . However, AAV-based delivery involves persistent expression of the Cas9 nuclease, a bacterial protein. Recent studies indicate a high prevalence of neutralizing antibodies and T cells specific to the commonly used Cas9 orthologs from Streptococcus pyogenes (SpCas9) and Staphylococcus aureus (SaCas9) in humans. We tested in a mouse model whether pre-existing immunity to SaCas9 would pose a barrier to liver genome editing with AAV packaging CRISPR-Cas9. Although efficient genome editing occurred in mouse liver with pre-existing SaCas9 immunity, this was accompanied by an increased proportion of CD8 + T cells in the liver. This cytotoxic T cell response was characterized by hepatocyte apoptosis, loss of recombinant AAV genomes, and complete elimination of genome-edited cells, and was followed by compensatory liver regeneration. Our results raise important efficacy and safety concerns for CRISPR-Cas9-based in vivo genome editing in the liver.