Channelrhodopsin: Potential Applications in Vision Restoration

Zhuo‐Hua Pan, Wayne State University School of Medicine, Detroit, Michigan, USA
Alexander M Dizhoor, Salus University, Elkins Park, Pennsylvania, USA

Published online: September 2009

DOI: 10.1002/9780470015902.a0021388

Channelrhodopsins, the directly light-gated ion channels from green algae, provide a powerful tool for basic research in neuroscience as well as potential applications for treating neurological diseases and disorders. One of the promising clinical applications is to cure blindness caused by the death of rod and cone photoreceptors.

Key concepts:

  • Light, through multiple conformational changes caused by reversible isomerization of retinal chromophore group, transiently creates ion conductance in microbial retinal binding proteins, channelrhodopsins.
  • Photocycle of channelrhodopsins remains preserved when they are heterologously expressed in animal and human cells.
  • Channelrhodoposins are directly light-gated cation channels with million second activation and deactivation kinetics.
  • A sufficient number of functional channelrhodopsins can be formed in animal neurons, with endogenous chromophore groups as supplied by regular diet. Thus, only introducing a single gene of channelrhodopsins is required to turn various types of animal neurons into artificial photoreceptor cells.
  • Converting surviving inner retinal neurons into directly photosensitive cells is a new strategy to treating blindness after the death of rod and cone photoreceptors.
  • Using delivery by adeno-associated viral vectors, long-term expression of channelrhodopsin-2 can be achieved in rodent inner retinal neurons in vivo.
  • Expression of channelrhodopsin-2 in surviving inner retinal neurons of a mouse model with retinal degeneration can restore the ability of the retina to encode light signals and transmit the light signals to the visual cortex.

Keywords: channelrhodopsins; directly light-gated ion channels; retinal blinding diseases; vision restoration

Figure 1. Photocycle of ChR2. The D470 dark state is converted by a light-induced retinal isomerization into an early intermediate, P500. Thermal relaxation accompanied by transient Schiff base deprotonation (P390) and conformational changes of the protein creates the putative conducting state, P520. The D470 dark state recovers via the two P480 intermediates, P480a and P480b. The latter, via a major backbone rearrangement in P480b, leads to restoration of the original dark state. Like P520, the P480b intermediate is photoreactive and can be converted by light to the early P500 intermediate, but at lower quantum efficiency. Copyright 2008, by the American Society for Biochemistry and Molecular Biology. Modified from Ritter et al. (2008).
Figure 2. Expression of a Chop2-GFP fusion construct in HEK 293 cells. (a) pCMV-Chop2-GFP expression cassette. (b) Chop2-GFP expression in HEK 293 cells with pCMV-Chop2-GFP viewed under a fluorescence microscope. Scale bar: 15 m. (c) and (d) Representative recordings of light responses from Chop2-GFP fusion protein-expressing HEK 293 cells. The current was recorded from a HEK cell, preincubated in the presence of 1 M all-trans retinal (c). The current was also observed in HEK cells in the absence of exogenous all-trans retinal. (e) The average currents recorded from HEK cells in the absence or presence of 0.1 and 1 M all-trans retinal.
Figure 3. Properties of light-evoked currents of the ChR2-expressing retinal neurons (modified from Bi et al., 2006). (a) A recording of a Chop2-expressing retinal cell response to light stimuli of wavelengths ranging from 420 to 580 nm. (b) A representative recording of the currents elicited by light stimuli at a wavelength of 460 nm with light intensities ranging from 2.2×1015 to 1.8×1018 photons cm–2 s–1. (c) Light-intensity response curve. The data points were fitted with a single logistic function curve. Reprinted from Bi et al., 2006, with permission from Elsevier.
Figure 4. Expression and light-response properties of ChR2 in retinal neurons of rd1/rd1 mice (modified from Bi et al., 2006). Chop2-GFP fluorescence viewed in flat retinal whole-mount (a) and vertical sections (b). (c) Light microscope image of a semi-thin vertical retinal section. Scale bar: 50 m in (a) and 30 m in (b) and (c). (d) Representative recordings of a Chop2-GFP-positive neuron in four incremental intensities at a wavelength of 460 nm. The relationship between the light intensity and the current amplitude (e), membrane depolarization (f), and the number of spikes (g). Reprinted from Bi et al., 2006, with permission from Elsevier.
Figure 5. Central projections of Chop2-GFP-expressing retinal ganglion cells and visual-evoked potentials in rd1/rd1 mice (modified from Bi et al., 2006). GFP-labeled terminal arbours of retinal ganglion cells in ventral lateral geniculate nucleus and dorsal lateral geniculate nucleus (a) and in superior colliculus (b). OT, optical track; vLGN, ventral lateral geniculate nucleus; dLGN, dorsal lateral geniculate nucleus; SC, superior colliculus. Scale bar: 200 m in (a), 100 m in (b). (c) VEPs recorded from a wild-type mouse. The responses were observed at both wavelengths of 460 and 580 nm. (d) VEPs recorded from an rd1/rd1 mouse injected with Chop2-GFP viral vectors. The responses were elicited only by light at the wavelength of 460 nm but not at the wavelength of 580 nm. (e) No detectable VEPs were observed from rd1/rd1 mice injected with viral vectors carrying GFP alone. Reprinted from Bi et al., 2006, with permission from Elsevier.
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Article Title: Channelrhodopsin: Potential Applications in Vision Restoration
 
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Pan, Zhuo‐Hua, and Dizhoor, Alexander M(Sep 2009) Channelrhodopsin: Potential Applications in Vision Restoration. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021388]