Control of organelle biogenesis by the Lipin switch
Lipid metabolism is a tightly regulated cellular program that maintains lipid homeostasis and controls organelle biogenesis. By tuning lipid metabolism, cells decide whether lipids are used for the synthesis of organelle membranes or stored for future consumption. In eukaryotes, phosphatidate phosphatases of the Lipin family are important regulators of lipid metabolism with established links to human disease. Lipins act as molecular switches by oscillating between a phosphorylated inactive form in the cytosol and a dephosphorylated active form bound to organelle membranes, in particular the membrane of the endoplasmic reticulum (ER). When membrane-bound, Lipins convert phosphatidic acid into diacylglycerol, thereby channeling lipids towards stor- age in lipid droplets. The Schuck lab recently performed a genetic screen for regulators of ER membrane biogenesis in yeast and identified the ER protein Ice2. Together with previous work from other laboratories, this discovery led to the definition of a signaling cascade that controls Lipin activity. However, the physiological stimuli that initiate this cascade and the mechanisms that convert these stimuli into enhanced membrane biogenesis or lipid storage are incompletely understood, especially in mammalian cells. Furthermore, no structural information exists on any Lipin, so that the exact molecular mechanism of the Lipin switch remains unknown. To address these issues, we propose a multidisciplinary work program. In Aim 1, we will delineate stimuli that affect the Lipin switch in yeast and human tissue culture cells. Furthermore, we will genetically identify new molecular players upstream of Lipin, again both in yeast and human cells. In Aim 2, we will elucidate the mechanism of the Lipin switch by a combination of structural biology approaches. Through structure-function analyses, including biochemical activity and membrane binding assays, we will dissect the regulation of Lipin. In Aim 3, we will address the physiological consequences of Lipin activity. In collaboration with other laboratories of the Transregio, we will develop re-engineered Lipin switches, which will allow us to precisely control Lipin activity in space and time. We will use these artificial switches and structure-based mutants to define the role of Lipins in the biogenesis of the ER and lipid droplets. Thus, the proposed research will provide new insights into the regulation, mechanism and physiology of the Lipin switch. This work may ultimately enable the development of small molecule modulators of Lipins for therapeutic purposes.