The Energy Supply of the Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a spongiform, highly branched organelle of complex cells. As a central and vital cell organelle the ER regulates many processes in living cells. Dysfunctions of ER- related processes can lead to and maintain different diseases including neuro degeneration, diseases of the cardio-vascular system and cancer. Although many essential components and functions of the ER have been identified, little is known about the energy supply of this important cell organelle. While it is clear that the ER depends on the supply of adenosine triphosphate (ATP), the most important cellular transport form of energy, the molecules and processes that accomplish the transport of ATP to and into the ER are currently largely unidentified. In this project state-of-the-art methods in biochemistry, cell physiology, and cell biology are combined in order to exactly unravel the nature and significance of the energy supply of the ER. An essential technique for this issue is to monitor real-time changes of ATP within the ER. Recently, we were able to perform such measurements by the development of refined ATP probes that consist of fluorescent proteins and provide a specific and real-time readout of ER ATP dynamics in single cells. Based on this sophisticated technique we now plan to investigate how the energy supply of the ER can be manipulated specifically, how alterations of the ER ATP content impacts on cell functions, and if such approaches can be used for the development of novel strategies to successfully treat metabolic diseases, neuro degeneration and cancer.

The Energy Supply of the Endoplasmic Reticulum The endoplasmic reticulum (ER) is a vital cellular organelle, which fulfills multiple important cell functions that depend on an efficient energy supply. Particularly protein folding within the lumen of the ER requires energy supply in the form of adenosine triphosphate (ATP), which is generated within the cytosol and more efficiently within mitochondria, the cellular power plants. The nature and importance of the ER energy supply for cell functions, cell signaling and the cellular metabolic activity is, however, quite poorly understood, despite the assumption that any imbalances of this process might be related to the pathology of several diseases such as cancer. In this research project, we investigated the energy supply of the ER in cancer cells, during cell stress and ageing by using sophisticated fluorescent biosensors. Based on our preliminary data and the novel methods and protocols we have introduced, recently a protein, referred to as SLC35B1, could be identified as an ER ATP transporter in the mammalian system. Very recently, we could confirm the importance of this protein for ER ATP import and unveiled a strong dependency of its action from calcium ions (Ca2 ) in cancer cells. Our research further showed that the uptake of Ca2 by mitochondria as well as their vicinity to the ER is highly regulated in many cancer cell types. Interestingly, also the concentration of potassium ions at enzymes, which are import for the glucose metabolism on the surface of mitochondria, seem highly relevant for the energy metabolism of certain cancer cells. Adequate disturbances of such metabolic settings with different kind of chemical compounds might be used in future as new therapeutic approaches in the treatment of human cancer. In order to gain our understanding of cancer specific peculiarities, we developed novel fluorescent biosensors. These sophisticated probes such as the GEPIIs (genetically encoded fluorescent potassium ion indicators) now allow the very first time to visualize and investigate cell signaling and metabolism with high resolution and in a high content manner. We developed a new fluorescent pH probe, referred to as pH-Lemon, which allows investigating the activity of the acidic lysosomes, which are so important for autophagy and providing substrates for energy recycling. Recently, we also introduced mito-STAR, a tool to visualize the import of a certain protein, which regulates the cellular energy metabolism, into mitochondria or its abundance within the nucleus upon cell stress. With the help of such novel probes and tools we will be able to better understand the peculiarities of cancer cells in cell signaling and metabolism. This research has therefore the potency to come up with novel strategies that are more effective in the personalized diagnosis and therapy of human cancer.