Genetically encoded Ca²⁺-sensor reveals details of porcine endothelial cell activation upon contact with human serum

The activation of the endothelial surface in xenografts is still a poorly understood process and the consequences are unpredictable. The role of Ca²⁺-messaging during the activation of endothelial cells is well recognized and routinely measured by synthetic Ca²⁺-sensitive fluorophors. However, these compounds require fresh loading immediately before each experiment and in particular when grown in state-of-the-art 3D cell culture systems, endothelial cells are difficult to access with such sensors. Therefore, we developed transgenic pigs expressing a Ca²⁺-sensitive protein and examined its principal characteristics. Primary transgenic endothelial cells stimulated by ATP showed a definite and short influx of Ca²⁺ into the cytosol, whereas exposure to human serum resulted in a more intense and sustained response. Surprisingly, not all endothelial cells reacted identically to a stimulus, rather activation took place in adjacent cells in a timely decelerated way and with distinct intensities. This effect was again more pronounced when cells were stimulated with human serum. Finally, we show clear evidence that antibody binding alone significantly activated endothelial cells, whereas antibody depletion dramatically reduced the stimulatory potential of serum. Transgenic porcine endothelial cells expressing a Ca²⁺-sensor represent an interesting tool to dissect factors inducing activation of porcine endothelial cells after exposure to human blood or serum.