Ras Protein Transfer during Trans-Endothelial Migration

The direct exchange of membrane proteins between cells is an intriguing aspect of cell communication which has gained further importance by the recent discovery that membrane-associated signaling molecules like Ras are equally transferred and may elicit a functional signal in recipient cells. While intercellular protein transfer has primarily been investigated for antigen-presenting and effector cells of the immunological synapse, we have hypothesized that this process might be of particular relevance during diapedesis and we have gained initial evidence supporting this concept. When blood cells traverse the endothelial monolayer to extravasate into the surrounding tissue, they are in close contact with endothelial cells which may facilitate the exchange of membrane- associated proteins and promote a rapid signal of activation. We thus propose to investigate the exchange of functionally active Ras between endothelial cells and transmigrating cell populations in two distinct in vitro models: 1. Endothelial Ras transfer into leukocytes upon pro- inflammatory activation of endothelial cells and subsequent diapedesis of leukocytes. 2. Oncogenic Ras transfer onto endothelial cells when tumor cells traverse the endothelium in a metastatic process. The efficiency of transfer, the mode of action and the functional consequences of Ras exchange shall be analyzed in detail. By this investigation we aim to unravel a novel and important aspect of cell communication and deduce potential targets and mechanisms to manipulate the function of extravasating cells.

Contact between cells generally involves binding of surface proteins which triggers signals within the cells that lead to cell activation and to functional changes. In addition, cells can exchange whole patches of their surface membrane and thereby transfer proteins and signaling molecules to the interacting cell. The project "Ras protein transfer during trans-endothelial migration" has addressed the question whether the signal molecule Ras is exchanged when cells leave the blood stream to migrate into the tissue and whether the Ras transfer may lead to rapid cell activation. In this process the migrating cells have to traverse the blood vessel wall and are in close contact with vessel-lining endothelial cells. Based on a laboratory model we could show that Ras protein is indeed transferred from endothelial cells to the trans-migrating population. Furthermore, endothelial cells can take up Ras from interacting cells. We have characterized the underlying mechanism of Ras exchange. With respect to cell functions, the uptake of Ras protein from endothelium showed moderate effects of cell activation which are considered supplemental to cell communication via surface protein interactions.