The Problem

Producing complex therapeutic proteins often requires biosynthesis in mammalian cells. This method of expressing proteins presents many obstacles. Our project focuses on one: getting a mammalian cell to express the desired protein when we want it to. Currently, bioreactors in industry rely heavily on small signaling molecules to get the cells to respond. The time it takes for these molecules to diffuse through the cell culture medium and the subsequent removal they require when the product is purified are huge problems. To allow a finer control over gene expression our project studied the implementation of two light-induced genetic (or optogenetic) switches. Both allow a tight regulation of gene expression and eliminate the problem of removing signaling molecules during the final purification of the protein that was synthesized.

Many of the proteins synthesized for therapeutic purposes can also be toxic to the cells that produce them. A fine control of gene expression in these cases is crucial to the viability of the cells being cultivated. Small signaling molecules aren't ideal for these purposes but light is instantaneous and easy to vary.

The Simple Switch

The first switch we studied is an untested fusion protein designed to act as a light-induced transcriptional activator in mamallian cells. The LovTAP-VP16 protein consists of a Lov2 domain (from ) a Trp repressor (from ) and a VP16 transactional activating domain (from ). A first version of this fusion protein was developed at the university of Chicago and later characterized and used as a bacterial repressor by the EPFL 2009 iGEM. The protein binds to a TRP promoter after undergoing a conformational change when exposed to light. By fusing the Lov-TAP protein to a viral promoter, the idea was to turn the protein into a light activated DNA binding protein. When the TRP promoter is bound to the LovTAP-VP16 protein the viral promoter on the C terminal recruits RNA polymerase II to the site and favors the transcription of the reporter gene next to the TRP promoter.

This pathway is simple and light activation of LovTAP-VP16 results in direct activation of transcription. There are also major obstacles this approach presents though. The protein needs to be localized in the nucleus, bind well to DNA and long enough to activate transcription.

For this system we will be using a DsRed readout to characterize the speed and efficiency of transcription by quantifying fluorescence.

The Complex Switch

In addition to LovTAP switch, we will be realizing another, more complex, melanopsin-based light switch developed by Fussennegger et al. (Science 332, 2011). In this switch, a light-sensitive membrane bound protein, melanopsin, is inserted to trigger a cascade. The melanopsin opens calcium channels in the cell which activates the NFAT pathway. By inserting a readout gene next to an NFAT promoter, we can promote its expression by triggering the release of calcium with light. In our expriments we used GFP as our readout protein for the same reasons DsRed was used in the LovTAP-VP16 experiments. This optogenetic switch takes advantage of an already existing mammallian pathway and limits the potential flaws related to promoting transcription once the pathway is activated. However, calcium is a broad effector and can have unintended consequences. Different cell types also react differently to calcium influx and this approach might not be generalizable. Fussenegger's team were successful promoting expression in HEK (human embryo kidney) cells, and we will also try to get the pathway to work in CHO cells and compare the results to the work done in HEK cells.

Constructs

Describe the different plasmids.

Future Work

Characterizing and troubleshooting the activating characteristics of the LovTAP-VP16 protein should be a priority in getting the simple switch system to work. It's still not clear whether or not it is localized in the nucleus and properly binds DNA with the additional VP16 promoter domain.

A nuclear exctract with western blotting or immunofluorescence techniques could be used to localise the protein. If the localisation of the protein isn't correct the nuclear localisation signal might need modification.

A Chromatin immunoprecipitation assay could be used to verify the binding characteristics of th LovTAP protein in the case where it is localised in the nucleus but still doesnt work.

Lastly, if the protein can bind DNA and is localized in the nuclues, a different promoter/linker combination could be used instead of VP16 to initiate transcription.

Other verions of LovTAP which respond better to light have also been characterized by Strickland et al.

[http://www.nature.com/nmeth/journal/v7/n8/full/nmeth.1473.html Improvement of Lov domain based photoswitches]

By using an improved version of the protein with a more dynamic response to light we could improve the DNA binding response to light and activation of trancsription.

Lastly, the creation of stable cell lines for each of our expression systems would make since for a longer term project.