Faculty Bibliography

The esterification of amphiphilic alcohols with fatty acids is a ubiquitous strategy implemented by eukaryotes and some prokaryotes to conserve energy and membrane progenitors and simultaneously detoxify fatty acids and other lipids. This key reaction is performed by at least four evolutionarily unrelated multigene families. The synthesis of this "neutral lipid" leads to the formation of a lipid droplet, which despite the clear selective advantage it confers is also a harbinger of cellular and organismal malaise. Neutral lipid deposition as a cytoplasmic lipid droplet may be thermodynamically favored but nevertheless is elaborately regulated. Optimal utilization of these resources by lipolysis is similarly multigenic in determination and regulation. We present here a perspective on these processes that originates from studies in model organisms, and we include our thoughts on interventions that target reductions in neutral lipids as therapeutics for human diseases such as obesity and diabetes.

The PAH1-encoded phosphatidate (PA) phosphatase in Saccharomyces cerevisiae is a pivotal enzyme that produces diacylglycerol for the synthesis of triacylglycerol (TAG) and simultaneously controls the level of PA used for phospholipid synthesis. Quantitative lipid analysis showed that the pah1Delta mutation caused a reduction in TAG mass and an elevation in the mass of phospholipids and free fatty acids, changes that were more pronounced in the stationary phase. The levels of unsaturated fatty acids in the pah1Delta mutant were unaltered, although the ratio of palmitoleic acid to oleic acid was increased with a similar change in the fatty acid composition of phospholipids. The pah1Delta mutant exhibited classic hallmarks of apoptosis in stationary phase and a marked reduction in the quantity of cytoplasmic lipid droplets. Cells lacking PA phosphatase were sensitive to exogenous fatty acids in the order of toxicity palmitoleic acid > oleic acid > palmitic acid. In contrast, the growth of wild type cells was not inhibited by fatty acid supplementation. In addition, wild type cells supplemented with palmitoleic acid exhibited an induction in PA phosphatase activity and an increase in TAG synthesis. Deletion of the DGK1-encoded diacylglycerol kinase, which counteracts PA phosphatase in controlling PA content, suppressed the defect in lipid droplet formation in the pah1Delta mutant. However, the sensitivity of the pah1Delta mutant to palmitoleic acid was not rescued by the dgk1Delta mutation. Overall, these findings indicate a key role of PA phosphatase in TAG synthesis for protection against fatty acid-induced toxicity.

Behavioural decisions involving foraging, mate choice and habitat selection are complex and difficult to evaluate experimentally. Web abandonment by orb spiders is a complex decision that is experimentally tractable. For females of the golden orb spider, Nephila clavipes, the web is a microhabitat that serves as a prey capture device, a mating site and a habitat for parasites. Thus, the web embodies the complexity of a bird territory or mammal home range, but is spatially compact and amenable to experimental manipulation. We used both field census data and field experimental manipulations to address the importance of prey capture rate, kleptoparasite load and male presence for web tenure (the time spent at a web site) in both mature and immature female N. clavipes. No factor explained variation in web tenure for immature females, although census data suggested that increased kleptoparasite load decreased web tenure. For mature females, increased male presence decreased web tenure, while increased prey capture rate, condition and body size all increased web tenure. Web tenure also decreased over the course of the season. Females integrate multiple cues to make web movement decisions. One of these cues is male presence, which detracts from the quality of a web site, suggesting that mate harassment might affect females' web movement decisions. Insight into this seemingly simple behaviour contributes to a growing understanding of how and when animals integrate multiple cues into behavioural decisions. (C) 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Deletion of the acyltransferases responsible for triglyceride and steryl ester synthesis in Saccharomyces cerevisiae serves as a genetic model of diseases where lipid overload is a component. The yeast mutants lack detectable neutral lipids and cytoplasmic lipid droplets and are strikingly sensitive to unsaturated fatty acids. Expression of human diacylglycerol acyltransferase 2 in the yeast mutants was sufficient to reverse these phenotypes. Similar to mammalian cells, fatty acid-mediated death in yeast is apoptotic and presaged by transcriptional induction of stress-response pathways, elevated oxidative stress, and activation of the unfolded protein response. To identify pathways that protect cells from lipid excess, we performed genetic interaction and transcriptional profiling screens with the yeast acyltransferase mutants. We thus identified diacylglycerol kinase-mediated phosphatidic acid biosynthesis and production of phosphatidylcholine via methylation of phosphatidylethanolamine as modifiers of lipotoxicity. Accordingly, the combined ablation of phospholipid and triglyceride biosynthesis increased sensitivity to saturated fatty acids. Similarly, normal sphingolipid biosynthesis and vesicular transport were required for optimal growth upon denudation of triglyceride biosynthesis and also mediated resistance to exogenous fatty acids. In metazoans, many of these processes are implicated in insulin secretion thus linking lipotoxicity with early aspects of pancreatic beta-cell dysfunction, diabetes, and the metabolic syndrome.