Regulating native ion channels with light
The PTL approach has the great benefit of specificity, but the expression of exogenous genes in neurons presents some major challenges. Genes can be introduced by chemically facilitated gene transfection or viral-mediated gene transfer, but successful gene expression varies considerably amongst neurons in a population and is slow, typically requiring days to weeks. For these reasons, we have sought a simple method for bestowing light-sensitivity onto neurons that does not require exogenous gene expression, but rather can be carried out on freshly obtained, relatively unadulterated, neural tissue. We have discovered a photoswitch molecule, named AAQ, that confers light-sensitivity on K+ channels without requiring covalent attachment. In fact, nearly every K+ channel involved in electrical excitability is photosensitized by AAQ, enbling widespread photosensitization of neural tissue. Neurons treated with AAQ for as little as 5 minutes become sensitive to light, with one wavelength (380 nm) unblocking K+ channels and suppressing firing, and another (500 nm) blocking the channels and promoting firing. We have shown that AAQ bestows light-sensitivity on neurons in culture, neurons in freshly obtained brain slices, and neurons in vivo (Fortin et al., Nature Methods, 2008).