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Let there be light-activated genes
Hybrid tool combines light with chemicals to fine-tune gene expression

Japan -- While each cell in your body generally contains the same DNA, whether that cell is in the skin on your nose or in a leg muscle depends on how its genes have been deciphered. This so-called 'gene expression' is a complex mechanism that adjusts dynamically depending on each cell's ultimate shape and function.

For some time now, researchers have had tools to artificially turn specific genes 'on' or 'off' at will.

And now, a team from Kyoto University's Graduate School of Biostudies have increased the precision of one tool by combining two well-established gene manipulation technologies: the 'Tetracycline-controlled transcriptional activation -- Tet -- system' and the 'light-activated protein interaction system'.

"Many researchers have developed tools that activate genes in different ways. Some are based on proteins that change shape when they are stimulated by light illumination," explains first author Mayumi Yamada, who published the study in Cell Reports.

"Once the protein shape has changed, it will attach to a specific part of a cell's DNA and then begin the process of expressing a gene to make a protein."

For mammalian cells, the Tet system is one such common, chemically-regulated system that can activate specific genes. It utilizes a small molecule called Doxycyclin -- or Dox -- to move the Tet transcription activator on or off of the DNA, thereby controlling gene expression.

Alternatively, light sensitive proteins -- that change shape in response to particular wavelengths -- have been used to activate genes in yeast. Their efficiency in mammalian cells is low, however, and even the Tet system is not perfect, due to the difficulty of fine spatio-temporal regulation, clouding results.

"These drawbacks can limit the accuracy of our data," continues Yamada, "because with Tet we can't turn a gene on and then immediately off again since Dox is still present."

"Realizing this, our team came upon the idea of controlling the Tet system using light, to make a 'Photo-Activated Tet transcription activator', or PA-Tet."

The researchers took the key elements of both protein tools and designed them to bind together in the presence of blue light, resulting in two versions of PA-Tet: PA-Tet-ON and PA-Tet-OFF. In this case of the PA-Tet-ON system, the Tet element activates with Dox, but when the light is turned off, the protein complex splits apart and ceases all activity.

This new tool was shown to increase or decrease activity depending on the concentration of Dox as well as due to light intensity, and even short bursts of light could pulse gene expression levels.

"We can now accurately control gene expression inside a living mouse's body, such as in the brain and under the skin," states Yamada.

Lead researcher Itaru Imayoshi concludes, "Our lab's main focus is neural development and regeneration of the brain. Neural stem cells show dynamic changes in gene expression in a precise and timed manner. This PA-Tet system will help not only our field of research but the wider study of genetics and cell biology."

Yamada, M., Suzuki, Y., Nagasaki, S., Okuno, H. and *Imayoshi, I. (2018) Light-inducible Tet-gene expression system in mammalian cells. Cell Reports 25, 487-500.

doi: 10.1016/j.celrep.2018.09.026.