Surface features of the lipid droplet mediate perilipin 2 localization.

All eukaryotic organisms store excess lipid in intracellular lipid droplets. These dynamic structures are associated with and regulated by numerous proteins. Perilipin 2, an abundant protein on most lipid droplets, promotes neutral lipid accumulation in lipid droplets. However, the mechanism by which perilipin 2 binds to and remains anchored on the lipid droplet surface is unknown. Here we identify features of the lipid droplet surface that influence perilipin 2 localization. We show that perilipin 2 binding to the lipid droplet surface requires both hydrophobic and electrostatic interactions. Reagents that disrupt these interactions also decrease binding. Moreover, perilipin 2 binding does not depend on other lipid droplet-associated proteins but is influenced by the lipid composition of the surface. Perilipin 2 binds to synthetic vesicles composed of dioleoylphosphatidylcholine, a phospholipid with unsaturated acyl chains. Decreasing the temperature of the binding reaction, or introducing phospholipids with saturated acyl chains, decreases binding. We therefore demonstrate a role for surface lipids and acyl chain packing in perilipin 2 binding to lipid droplets. The ability of the lipid droplet phospholipid composition to impact protein binding may link changes in nutrient availability to lipid droplet homeostasis.

Changing the phospholipid composition of lipid droplets alters localization of select lipid droplet proteins.

Our experiments aim to determine if decreasing the amount of phosphatidylcholine (PC) relative to phosphatidylethanolamine (PE) at the lipid droplet surface changes the localization of specific lipid droplet proteins. We manipulate lipid droplet phospholipids in both a cultured mouse hepatocyte (AML12) cell line and on synthetic lipid droplets. Decreasing the PC:PE ratio increases perilipin 2, decreases DGAT2, and does not change rab18 or lanosterol synthase levels on lipid droplets. These differences may be explained by the distinct structural motifs that mediate the protein-lipid droplet interactions.

A critical role for eukaryotic elongation factor 1A-1 in lipotoxic cell death.

The deleterious consequences of fatty acid (FA) and neutral lipid accumulation in nonadipose tissues, such as the heart, contribute to the pathogenesis of type 2 diabetes. To elucidate mechanisms of FA-induced cell death, or lipotoxicity, we generated Chinese hamster ovary (CHO) cell mutants resistant to palmitate-induced death and isolated a clone with disruption of eukaryotic elongation factor (eEF) 1A-1. eEF1A-1 involvement in lipotoxicity was confirmed in H9c2 cardiomyoblasts, in which small interfering RNA-mediated knockdown also conferred palmitate resistance. In wild-type CHO and H9c2 cells, palmitate increased reactive oxygen species and induced endoplasmic reticulum (ER) stress, changes accompanied by increased eEF1A-1 expression. Disruption of eEF1A-1 expression rendered these cells resistant to hydrogen peroxide- and ER stress-induced death, indicating that eEF1A-1 plays a critical role in the cell death response to these stressors downstream of lipid overload. Disruption of eEF1A-1 also resulted in actin cytoskeleton defects under basal conditions and in response to palmitate, suggesting that eEF1A-1 mediates lipotoxic cell death, secondary to oxidative and ER stress, by regulating cytoskeletal changes critical for this process. Furthermore, our observations of oxidative stress, ER stress, and induction of eEF1A-1 expression in a mouse model of lipotoxic cardiomyopathy implicate this cellular response in the pathophysiology of metabolic disease.

Using molecular visualization to explore protein structure and function and enhance student facility with computational tools.

Recognizing that undergraduate students can benefit from analysis of 3D protein structure and function, we have developed a multiweek, inquiry-based molecular visualization project for Biochemistry I students. This project uses a virtual model of cyclooxygenase-1 (COX-1) to guide students through multiple levels of protein structure analysis. The first assignment explores primary structure by generating and examining a protein sequence alignment. Subsequent assignments introduce 3D visualization software to explore secondary, tertiary, and quaternary structure. Students design an inhibitor, based on scrutiny of the enzyme active site, and evaluate the fit of the molecule using computed binding energies. In the last assignment, students introduce a point mutation to model the active site of the related COX-2 enzyme and analyze the impact of the mutation on inhibitor binding. With this project we aim to increase knowledge about, and confidence in using, online databases and computational tools. Here, we share results of our mixed methods pre- and postsurvey demonstrating student gains in knowledge about, and confidence using, online databases and computational tools. © 2017 by The International Union of Biochemistry and Molecular Biology, 45(4):318-328, 2017.

An expanded framework for biomolecular visualization in the classroom: Learning goals and competencies.

A thorough understanding of the molecular biosciences requires the ability to visualize and manipulate molecules in order to interpret results or to generate hypotheses. While many instructors in biochemistry and molecular biology use visual representations, few indicate that they explicitly teach visual literacy. One reason is the need for a list of core content and competencies to guide a more deliberate instruction in visual literacy. We offer here the second stage in the development of one such resource for biomolecular three-dimensional visual literacy. We present this work with the goal of building a community for online resource development and use. In the first stage, overarching themes were identified and submitted to the biosciences community for comment: atomic geometry; alternate renderings; construction/annotation; het group recognition; molecular dynamics; molecular interactions; monomer recognition; symmetry/asymmetry recognition; structure-function relationships; structural model skepticism; and topology and connectivity. Herein, the overarching themes have been expanded to include a 12th theme (macromolecular assemblies), 27 learning goals, and more than 200 corresponding objectives, many of which cut across multiple overarching themes. The learning goals and objectives offered here provide educators with a framework on which to map the use of molecular visualization in their classrooms. In addition, the framework may also be used by biochemistry and molecular biology educators to identify gaps in coverage and drive the creation of new activities to improve visual literacy. This work represents the first attempt, to our knowledge, to catalog a comprehensive list of explicit learning goals and objectives in visual literacy. © 2016 by The International Union of Biochemistry and Molecular Biology, 45(1):69-75, 2017.

Fluorescent Detection of Lipid Droplets and Associated Proteins.

Excess lipid is stored in intracellular organelles known as lipid droplets. This unit discusses techniques for the visualization of lipid droplets and associated proteins in cultured mammalian cells. Protocols for the detection of lipid droplets in fixed or live cells with BODIPY 493/503 are included. The best method for combining visualization of intracellular lipid droplets with indirect immunofluorescent detection of lipid droplet-associated proteins is described. Techniques for sample fixation and permeabilization must be chosen carefully to avoid alterations to lipid droplet morphology. Immunofluorescent detection of perilipin 2, a broadly expressed, lipid droplet-associated protein, widely used as a marker for lipid droplet accumulation, is presented as an example. Finally, a simple protocol for enhancing lipid droplet accumulation through supplementation with excess fatty acid is included. © 2016 by John Wiley & Sons, Inc.

Mechanisms of lipoapoptosis: implications for human heart disease.

Accumulation of long-chain fatty acids in the heart has been proposed to play a role in the development of heart failure and diabetic cardiomyopathy. Several animal models with increased cardiomyocyte lipid accumulation suggest a link between the accumulation of lipid, cardiomyocyte cell death and the development of cardiomyopathy. In this review, we discuss the mechanism through which fatty acid accumulation may contribute to the development or progression of heart failure by initiation of apoptotic cell death. Long-chain saturated fatty acids induce apoptosis through a mechanism involving the generation of reactive intermediates. Reactive intermediate production occurs in concert with de novo ceramide synthesis, but ceramide production is not required for cell death. Cardiomyocyte dysfunction and death from reactive intermediates generated by long-chain saturated fatty acids may contribute to the pathogenesis of human heart disease.

Small nucleolar RNAs U32a, U33, and U35a are critical mediators of metabolic stress.

Lipotoxicity is a metabolic stress response implicated in the pathogenesis of diabetes complications and has been shown to involve lipid-induced oxidative stress. To elucidate the molecular mechanisms of lipotoxicity, we used retroviral promoter trap mutagenesis to isolate a cell line that is resistant to lipotoxic and oxidative stress. We show that loss of three box C/D small nucleolar RNAs (snoRNAs) encoded in the ribosomal protein L13a (rpL13a) locus is sufficient to confer resistance to lipotoxic and oxidative stress in vitro and prevents the propagation of oxidative stress in vivo. Our results provide evidence for a previously unappreciated, non-canonical role for box C/D snoRNAs as regulators of metabolic stress response pathways in mammalian cells.

The non-coding RNA gadd7 is a regulator of lipid-induced oxidative and endoplasmic reticulum stress.

In obesity and diabetes, an imbalance in fatty acid uptake and fatty acid utilization leads to excess accumulation of lipid in non-adipose tissues. This lipid overload is associated with cellular dysfunction and cell death, which contribute to organ failure, a phenomenon termed lipotoxicity. To elucidate the molecular mechanism of lipid-mediated cell death, we generated and characterized a mutant Chinese hamster ovary cell line that is resistant to palmitate-induced cell death. In this mutant, random insertion of a retroviral promoter trap has disrupted the gene for the non-coding RNA, growth arrested DNA-damage inducible gene 7 (gadd7). Here we report that gadd7 is induced by lipotoxic stress in a reactive oxygen species (ROS)-dependent fashion and is necessary for both lipid- and general oxidative stress-mediated cell death. Depletion of gadd7 by mutagenesis or short hairpin RNA knockdown significantly reduces lipid and non-lipid induced ROS. Furthermore, depletion of gadd7 delays and diminishes ROS-induced endoplasmic reticulum stress. Together these data are the first to implicate a non-coding RNA in a feed-forward loop with oxidative stress and its induction of the endoplasmic reticulum stress response.

Fluorescent detection of lipid droplets and associated proteins.

Most eukaryotic cells can store excess lipid in cytosolic lipid droplets. This unit discusses techniques for the visualization of lipid droplets and associated proteins in cultured mammalian cells. Protocols for the detection of lipid droplets with nile red and BODIPY 493/503 are included. The differences in the spectral properties of these two lipophilic dyes and advantages of each are discussed. The best method for combining visualization of intracellular lipid droplets with indirect immunofluorescent detection of lipid droplet-associated proteins is described. Techniques for sample fixation and permeabilization must be chosen carefully to avoid alterations to lipid droplet morphology. Immunofluorescent detection of adipophilin, a broadly expressed, lipid droplet-associated protein, widely used as a marker for lipid droplet accumulation, is presented as an example. Finally, a simple protocol for enhancing lipid droplet accumulation through supplementation with excess fatty acid is included.

Adipocyte differentiation-related protein reduces the lipid droplet association of adipose triglyceride lipase and slows triacylglycerol turnover.

Although neutral lipid storage droplets are ubiquitous in eukaryotic cells, very little is known about how their synthesis and turnover are controlled. Adipocyte differentiation-related protein (ADRP; also known as adipophilin) is found on the surface of lipid droplets in most mammalian cell types. To learn how ADRP affects lipid storage, we stably expressed the protein in human embryonic kidney 293 (HEK 293) cells, which express little endogenous ADRP. As expected, ADRP was targeted to the surface of lipid droplets and caused an increase in triacylglycerol (TAG) mass under both basal and oleate-supplemented conditions. At least part of the increased mass resulted from a 50% decrease in the rate of TAG hydrolysis in ADRP-expressing cells. Furthermore, ADRP expression increased the fraction of total cellular TAG that was stored in lipid droplets. ADRP expression induced a striking decrease in the association of adipose triglyceride lipase (ATGL) and mannose-6-phosphate receptor tail-interacting protein of 47 kDa with lipid droplets and also decreased the lipid droplet association of several other unknown proteins. Transient expression of ADRP in two other cell lines also reduced the lipid droplet association of catalytically inactive ATGL. We conclude that the reduced lipid droplet association of ATGL and/or other lipases may explain the decrease in TAG turnover observed in ADRP-expressing HEK 293 cells.

Oligomerization of the murine fatty acid transport protein 1.

The 63-kDa murine fatty acid transport protein 1 (FATP1) was cloned on the basis of its ability to augment fatty acid import when overexpressed in mammalian cells. The membrane topology of this integral plasma membrane protein does not resemble that of polytopic membrane transporters for other substrates. Western blot analysis of 3T3-L1 adipocytes that natively express FATP1 demonstrate a prominent 130-kDa species as well as the expected 63-kDa FATP1, suggesting that this protein may participate in a cell surface transport protein complex. To test whether FATP1 is capable of oligomerization, we expressed functional FATP1 molecules with different amino- or carboxyl-terminal epitope tags in fibroblasts. These epitope-tagged proteins also form apparent higher molecular weight species. We show that, when expressed in the same cells, differentially tagged FATP1 proteins co-immunoprecipitate. The region between amino acid residues 191 and 475 is sufficient for association of differentially tagged truncated FATP1 constructs. When wild type FATP1 and the non-functional s250a FATP1 mutant are co-expressed in COS7 cells, mutant FATP1 has dominant inhibitory function in fatty acid uptake assays. Taken together, these results are consistent with a model in which FATP1 homodimeric complexes play an important role in cellular fatty acid import.

Triglyceride accumulation protects against fatty acid-induced lipotoxicity.

Excess lipid accumulation in non-adipose tissues is associated with insulin resistance, pancreatic beta-cell apoptosis and heart failure. Here, we demonstrate in cultured cells that the relative toxicity of two common dietary long chain fatty acids is related to channeling of these lipids to distinct cellular metabolic fates. Oleic acid supplementation leads to triglyceride accumulation and is well tolerated, whereas excess palmitic acid is poorly incorporated into triglyceride and causes apoptosis. Unsaturated fatty acids rescue palmitate-induced apoptosis by channeling palmitate into triglyceride pools and away from pathways leading to apoptosis. Moreover, in the setting of impaired triglyceride synthesis, oleate induces lipotoxicity. Our findings support a model of cellular lipid metabolism in which unsaturated fatty acids serve a protective function against lipotoxicity though promotion of triglyceride accumulation.

Prediction of novel Bag-1 homologs based on structure/function analysis identifies Snl1p as an Hsp70 co-chaperone in Saccharomyces cerevisiae.

Polypeptide binding by the chaperone Hsp70 is regulated by its ATPase activity, which is itself regulated by co-chaperones including the Bag domain nucleotide exchange factors. Here, we tested the functional contribution of residues in the Bag domain of Bag-1M that contact Hsp70. Two point mutations, E212A and E219A, partially reduced co-chaperone activity, whereas the point mutation R237A completely abolished activity in vitro. Based on the strict positional conservation of the Arg-237 residue, several Bag domain proteins were predicted from various eukaryotic genomes. One candidate, Snl1p from Saccharomyces cerevisiae, was confirmed as a Bag domain co-chaperone. Snl1p bound specifically to the Ssa and Ssb forms of yeast cytosolic Hsp70, as revealed by two-hybrid screening and co-precipitations from yeast lysate. In vitro, Snl1p also recognized mammalian Hsp70 and regulated the Hsp70 ATPase activity identically to Bag-1M. Point mutations in Snl1p that disrupted the conserved residues Glu-112 and Arg-141, equivalent to Glu-212 and Arg-237 in Bag-1M, abolished the interaction with Hsp70 proteins. In live yeast, mutated Snl1p could not substitute for wild-type Snl1p in suppressing the lethal defect caused by truncation of the Nup116p nuclear pore component. Thus, Snl1p is the first Bag domain protein identified in S. cerevisiae, and its interaction with Hsp70 is essential for biological activity.