👤 Saskia B Neher

Endothelial lipase (EL) is a key regulator of high-density lipoprotein (HDL) metabolism. Many aspects of EL function remain incompletely understood due to challenges in purifying active EL. This study identifies apolipoprotein J (ApoJ) as a novel chaperone for EL, crucial for its solubility and activity. Using an optimized purification protocol that yields active EL, we discovered that ApoJ consistently co-purifies with EL, maintaining its activity. We further show that knocking down ApoJ decreases the activity of EL. We demonstrate that ApoJ interacts with EL via its hydrophobic lid and tryptophan loop regions, and that mutating these regions abolishes the effect of ApoJ on the solubility and activity of EL. We show that ApoJ, EL, and ApoA1 (the defining lipoprotein of HDL particles) colocalize in HDL particles in mouse plasma. However, we find that ApoJ is not a direct carrier for EL to HDL particles. Instead, our data suggest that ApoJ primarily serves to enhance EL activity through its role as a chaperone, even when incorporated into lipid substrates. Our findings suggest a model in which ApoJ protects EL in plasma and enhances its hydrolysis of lipoprotein substrates. We propose that ApoJ is an accessory protein for EL, analogous to GPIHBP1 for LPL and co-lipase for PL. Further study of the interaction between EL and ApoJ will promote a better understanding of HDL metabolism. Show less

Lipoprotein lipase (LPL) is a triglyceride lipase that is contained in intracellular vesicles in an inactive storage form before secretion, but the precise structural details have not yet been resolved. Using cryo-electron tomography (cryo-ET), we observe that LPL exists inside of storage vesicles as a filament with an 11-nanometer diameter and is packed in these vesicles in two distinct patterns. Next, we solved a 4.2-Å resolution cryo-electron microscopy (cryo-EM) structure of this 11-nanometer LPL filament using purified protein. The filament is made of repeating pairs of LPL molecules with occluded active sites, rendering the LPL inactive. The comparison of the in situ subtomogram average and the in vitro cryo-EM structure indicates that the previously uncharacterized physiological storage form of LPL is an inactive filament. Show less

Lipoprotein lipase (LPL) is a critical enzyme in humans that provides fuel to peripheral tissues. LPL hydrolyzes triglycerides from the cores of lipoproteins that are circulating in plasma and interacts with receptors to mediate lipoprotein uptake, thus directing lipid distribution via catalytic and non-catalytic functions. Functional losses in LPL or any of its myriad of regulators alter lipid homeostasis and potentially affect the risk of developing cardiovascular disease-either increasing or decreasing the risk depending on the mutated protein. The extensive LPL regulatory network tunes LPL activity to allocate fatty acids according to the energetic needs of the organism and thus is nutritionally responsive and tissue dependent. Multiple pharmaceuticals in development manipulate or mimic these regulators, demonstrating their translational importance. Another facet of LPL biology is that the oligomeric state of the enzyme is also central to its regulation. Recent structural studies have solidified the idea that LPL is regulated not only by interactions with other binding partners but also by self-associations. Here, we review the complexities of the protein-protein and protein-lipid interactions that govern LPL structure and function. Show less

Elevated plasma triglycerides are a risk factor for coronary artery disease, which is the leading cause of death worldwide. Lipoprotein lipase (LPL) reduces triglycerides in the blood by hydrolyzing them from triglyceride-rich lipoproteins to release free fatty acids. LPL activity is regulated in a nutritionally responsive manner by macromolecular inhibitors including angiopoietin-like proteins 3 and 4 (ANGPTL3 and ANGPTL4). However, the mechanism by which ANGPTL3 inhibits LPL is unclear, in part due to challenges in obtaining pure protein for study. We used a new purification protocol for the N-terminal domain of ANGPTL3, removing a DNA contaminant, and found DNA-free ANGPTL3 showed enhanced inhibition of LPL. Structural analysis showed that ANGPTL3 formed elongated, flexible trimers and hexamers that did not interconvert. ANGPTL4 formed only elongated flexible trimers. We compared the inhibition of ANGPTL3 and ANGPTL4 using human very-low-density lipoproteins as a substrate and found both were noncompetitive inhibitors. The inhibition constants for the trimeric ANGPTL3 (7.5 ± 0.7 nM) and ANGPTL4 (3.6 ± 1.0 nM) were only 2-fold different. Heparin has previously been reported to interfere with ANGPTL3 binding to LPL, so we questioned if the negatively charged heparin was acting in a similar fashion to the DNA contaminant. We found that ANGPTL3 inhibition is abolished by binding to low-molecular-weight heparin, whereas ANGPTL4 inhibition is not. Our data show new similarities and differences in how ANGPTL3 and ANGPTL4 regulate LPL and opens new avenues of investigating the effect of heparin on LPL inhibition by ANGPTL3. Show less

Cardiovascular disease has been the leading cause of death throughout the world for nearly 2 decades. Hypertriglyceridemia affects more than one-third of the population in the United States and is an independent risk factor for cardiovascular disease. Despite the frequency of hypertriglyceridemia, treatment options are primarily limited to diet and exercise. Lipoprotein lipase (LPL) is an enzyme responsible for clearing triglycerides from circulation, and its activity alone can directly control plasma triglyceride concentrations. Therefore, LPL is a good target for triglyceride-lowering therapeutics. One approach for treating hypertriglyceridemia may be to increase the amount of enzymatically active LPL by preventing its inhibition by angiopoietin-like protein 4 (ANGPTL4). However, little is known about how these two proteins interact. Therefore, we used hydrogen-deuterium exchange MS to identify potential binding sites between LPL and ANGPTL4. We validated sites predicted to be located at the protein-protein interface by using chimeric variants of LPL and an LPL peptide mimetic. We found that ANGPTL4 binds LPL near the active site at the lid domain and a nearby α-helix. Lipase lid domains cover the active site to control both enzyme activation and substrate specificity. Our findings suggest that ANGPTL4 specifically inhibits LPL by binding the lid domain, which could prevent substrate catalysis at the active site. The structural details of the LPL-ANGPTL4 interaction uncovered here may inform the development of therapeutics targeted to disrupt this interaction for the management of hypertriglyceridemia. Show less