A fundamental feature of eukaryotic cells is the synthesis, trafficking, and localization of protein and lipid components into separate and distinct membrane compartments. Our lab studies how lipid metabolism is coordinately regulated with signaling pathways to achieve membrane lipid homeostasis.
We study the metabolism that controls the synthesis and turnover of inositol-containing phospholipids, which are prominent players in lipid-mediated signaling, membrane identity and membrane trafficking. Specifically, we study how changes in synthesis and turnover of lipids activate signaling networks, which provide feedback in regulating lipid metabolism itself.
We have shown that signals arising during ongoing lipid metabolism in the endoplasmic reticulum and plasma membrane regulate major transcriptional networks in the cell. Among the major signal transduction pathways influenced by changes in lipid metabolism are the unfolded protein response pathway, the protein kinase C pathway and the glucose response pathway. A number of mutants defective in these same pathways have been shown to exhibit a requirement for supplementation of their growth medium with inositol, a precursor of a number of essential phospholipids.
Our ongoing research is focused on discovering the mechanisms by which specific lipid metabolites activate signaling pathways and transcriptional networks in yeast. To accomplish this, we use the strategy of correlating changes in signaling and lipid metabolism that accompany the introduction or removal of the phospholipid precursor, inositol, from the medium of actively diving cells. Research conducted in our laboratory has shown that rapid fluctuations in the availability of exogenous inositol trigger changes in both lipid metabolism and the transcription of hundreds of genes. Statistical analysis identified at least six distinct expression responses that are triggered by addition of inositol. These regulatory responses to inositol include repression of phospholipid biosynthetic genes regulated by the Opi1p repressor and genes regulated by the unfolded response (UPR) pathway. Inositol addition also results in transient induction of lipid remodeling genes regulated by the transcription factor, Mga2p, which binds to the Low Oxygen Response Element (LORE). These three categories of genes are known to respond to signals arising in the ER and the kinetics of the changes in their transcript abundance are rapid, occurring within the first 15 to 30 minutes following introduction of inositol to the
The decline in PI levels following removal of inositol is also accompanied by alterations in the cellular pools of inositol containing lipids derived from PI-phosphates (PIPs) and inositol containing sphingolipids, and activation of protein kinase C (PKC) signaling. Moreover, mutants defective in PKC signaling exhibit inositol auxotrophy Fluorescent lipid-associated reporters (FLAREs), provided by the laboratory of Scott Emr, enabled the in vivo localization of pools of PI 4-phosphate (PI4P) and PI(4,5)P2, known to be involved in PKC signaling in yeast. The intensity of the PI4P FLARE at the PM increased in the absence of inositol and other conditions that activate PKC signaling. Tracing the changes in lipid metabolism related to this signal, we discovered that synthesis of the inositol SLs is reduced in inositol starved cells and that interruption of SL synthesis elicits both PKC signaling and the enrichment of the PI4P FLARE at the PM. The PI4P FLARE at the PM is highly correlated to activation of PKC signaling. However, it does not require the activity of components of the PKC signaling pathway downstream from the PI4P-5-kinase, Mss4p.growth medium of actively dividing cells. Analysis of changes in lipid metabolism over the same time frame revealed that PI synthesis increases rapidly after addition of inositol leading to rapid consumption of phosphatidic acid (PA), which interacts with the Opi1p repressor and is required for its retention in the ER. Thus, consumption of PA results in translocation of Opi1p to the nucleus, where it represses phospholipid biosynthetic genes, including INO1, encoding inositol 3-phosphate synthase. Increases in hydrolysis of phosphatidylcholine (PC) and triacylglycerol (TAG), in addition to de novo fatty acid synthesis, are also required to supply the fatty acids required for the burst of PI synthesis that occurs following addition of inositol. Conversely, the decline in PI levels following removal of inositol from the growth medium results in an increase in both PA and TAG levels.
Ongoing research in our laboratory focuses on understanding the nature of metabolic signals triggered by changes in lipid metabolism and their interaction with additional major cellular signal transduction pathways and transcriptional networks in the cell.