Cells can signal to each other through countless mechanisms. Paracrine and endocrine signaling is typically conducted through chemical means, by molecules such as hormones, neurotransmitters, and growth factors. Such signaling is limited by several factors. Diffusion speed limits all forms of cell-cell signaling; a molecule can only travel as fast as the medium it flows in. The species of the cells is also limiting; interkingdom signaling (a term which was only coined within the past decade) only occurs in special instances such as quorum sensing. Finally, scientists are limited in their ability to control cell signaling in vivo. Precise, temporal control of signaling is extremely difficult and has only been recently demonstrated by a technique called optogenetics, where light-activated ion channels are expressed in neurons, allowing neuroscientists to drive or silence action potentials with application of light. With this in mind, we set out to demonstrate a new method of cell signaling: signaling with light. If cells were able to communicate with light, it would overcome the aforementioned limitations because light is transmitted instantaneously, it is nonselective so it can signal between different species, and researchers can precisely and easily mediate this signaling by applying external light. Our goal was to create a line of “Sender Cells” and “Receiver Cells”, where the Sender Cells produce light, and the Receiver Cells receive the signal and produce an instantaneous response.

The Penn iGEM team has developed “Sender” and “Receiver” Human Embryonic Kidney (HEK) 293T cells which send and receive blue light, respectively. To construct the Sender cell, we developed HEK 293T cells which express Renilla luciferase, a 480nm light emitting protein. The Receiver cell expresses Aequorin (another luciferase) and CatCh, an ion channel gated by 480nm light. Since Aequorin requires Calcium to luminesce, and CatCh is highly permeable to calcium, the opening of CatCh causes the receiver cell to effectively relay the blue light signal by producing light itself. We have characterized this system and have submitted an Aequorin BioBrick to the Registry. We believe that integrating light into the repertoire of cell signaling will eventually prove useful for researchers; a recent review in Science (Chow & Boyden, 2011) has described a growing area of research called Synthetic Physiology, which marries optogenetics and Synthetic Biology in perturbing synthetic and natural gene networks with light.

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