Abstract
Channelrhodopsin-2 (ChR2) has become a celebrated research tool and is considered a promising potential therapeutic for neurological disorders. While making its way into the clinic, concerns about the safety of chronic ChR2 activation have emerged; in particular as the high-intensity blue light illumination needed for ChR2 activation may be phototoxic. Here we set out to quantify for the first time the cytotoxic effects of chronic ChR2 activation. We studied the safety of prolonged illumination on ChR2(D156A)-expressing human melanoma cells as cancer cells are notorious for their resistance to killing. Three days of illumination eradicated the entire ChR2(D156A)-expressing cell population through mitochondria-mediated apoptosis, whereas blue light activation of non-expressing control cells did not significantly compromise cell viability. In other words, chronic high-intensity blue light illumination alone is not phototoxic, but prolonged ChR2 activation induces mitochondria-mediated apoptosis. The results are alarming for gain-of-function translational neurological studies but open the possibility to optogenetically manipulate the viability of non-excitable cells, such as cancer cells. In a second set of experiments we therefore evaluated the feasibility to put melanoma cell proliferation and apoptosis under the control of light by transdermally illuminating in vivo melanoma xenografts expressing ChR2(D156A). We show clear proof of principle that light treatment inhibits and even reverses tumor growth, rendering ChR2s potential tools for targeted light-therapy of cancers.
Highlights
In the last decade optogenetics has revolutionized the neurosciences enabling neuroscientists to link neural network activity with behavior and disease
To set high measures for the cytotoxicity tests, we expressed ChR2(D156A) in the human melanoma cell line BLM,[13] as melanoma is renowned for its resistance apoptosis.[14]
The cytotoxic effects are expected to be enhanced in neurons, as cancer cells are almost resistant to killing
Summary
In the last decade optogenetics has revolutionized the neurosciences enabling neuroscientists to link neural network activity with behavior and disease. Latter in particular fostered the development of optogenetic treatment protocols for potential use in the clinic. A channelrhodopsin-2 (ChR2)based therapy to recover vision in the blind has recently been approved for clinical trials (NCT02556736) and optogenetic deep brain stimulation for motor and mood disorders such as Parkinson’s and depression are currently under active investigation. Illumination alone did not have any significant effects on cell viability, indicating that phototoxicity is not of primary concern, but instead it appears to be the chronic depolarization, potentially combined with constant Ca2+ inflow into the cytoplasm mediated through ChR2(D156A) that cause the cytotoxic effects. Sparing healthy tissue from therapy exposure is a critical challenge in the treatment of cancer that could be overcome in an optogenetic therapy by localized photoactivation
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