Optogenetics

Optogenetics is the use of light-sensitive proteins, introduced into cells by gene transfer, to control the electrical activity of those cells with light. In neuroscience it made possible a specific experiment that had not been available before: activating or silencing one genetically defined population of neurons, in a behaving animal, on the timescale of a single action potential, while leaving neighbouring cells untouched. Electrical methods such as Deep brain stimulation cannot provide that combination of cell-type specificity and millisecond timing.¹

§How it works

The active ingredient is a microbial opsin, a membrane protein from algae, archaea or bacteria that absorbs light and moves ions across the membrane. Channelrhodopsin-2, from the green alga Chlamydomonas reinhardtii, opens a cation channel when illuminated with blue light, depolarizing the cell. Halorhodopsin, a chloride pump from a haloarchaeon, and archaerhodopsin, a proton pump, hyperpolarize the cell under yellow or green light and so silence it. All of them require the cofactor all-trans retinal. Vertebrate brain tissue happens to supply it in sufficient quantity, and without that biological accident the technique would not work in mammals.

An experiment has three parts. First the opsin gene is delivered to the target cells, usually by AAV vectors carrying a cell-type-specific promoter, or by crossing a transgenic animal line expressing Cre recombinase in a defined population with a Cre-dependent opsin construct. Light is then delivered by an optical fibre, an implanted micro-LED array, or, in transparent preparations, from outside. Activity is read out by behaviour, electrophysiology or fluorescent calcium imaging.

The engineered variants matter as much as the original discovery. Trafficking sequences improve membrane targeting, and faster mutants follow spike trains above 100 Hz. Step-function opsins stay open for minutes after a brief pulse. Red-shifted opsins absorb longer wavelengths that scatter less in tissue. Soma-targeted opsins restrict expression to the cell body, so that holographic two-photon illumination can address individual neurons without exciting passing axons.

§Development history

1. 1971-1979
   Precursors
   Bacteriorhodopsin is characterized as a light-driven proton pump. Francis Crick later argues that neuroscience needs a method to control one cell type at a time, and speculates that light might provide it.
2. 2002-2003
   Channelrhodopsins identified
   Georg Nagel, Peter Hegemann and colleagues characterize channelrhodopsin-1 and channelrhodopsin-2 from green algae as directly light-gated ion channels.
3. 2002
   First genetically targeted photostimulation
   Gero Miesenböck's group controls neurons using a multi-component invertebrate phototransduction system, establishing the concept before a single-gene tool existed.
4. 2005
   Optogenetics in neurons
   Boyden, Zhang, Deisseroth and colleagues show that channelrhodopsin-2 alone confers millisecond-precision light control on mammalian neurons.
5. 2007-2010
   Inhibition and colour range
   Halorhodopsin and archaerhodopsin provide optical silencing; red-shifted and faster variants broaden the toolkit.
6. 2012-2013
   Memory engrams manipulated
   Optogenetic reactivation of hippocampal cells labelled during learning elicits the corresponding memory in mice, and a false memory is created by pairing artificial reactivation with a new experience.
7. 2013-2021
   Recognition
   The Brain Prize, the Shaw Prize and the Lasker Basic Medical Research Award are given for the development of optogenetics.
8. 2021
   First human result
   A patient with retinitis pigmentosa recovers partial visual function after retinal ganglion cells are made light-sensitive and driven by image-projecting goggles.

§What it is used for

In practice optogenetics is a research tool, and its influence on neuroscience comes from turning correlational claims into causal ones. Before it, showing that a population of neurons was active during a behaviour was straightforward, but showing that the activity caused the behaviour was not. Optogenetics supplies the missing manipulation. It has been used to map circuits for fear, reward, feeding, aggression, thirst, sleep-wake transitions and locomotion, generally by activating or silencing a projection defined by both its cell type and its anatomical target.

The most striking demonstrations concern memory. Cells active during fear learning can be tagged with an opsin using activity-dependent promoters, and reactivating those cells later elicits the behavioural signature of the memory without the original cue.² Pairing artificial reactivation of a context representation with an aversive stimulus produces behaviour consistent with a memory of an event that never occurred. These experiments are the strongest existing evidence that a specific memory has a discrete physical substrate that can be manipulated. That bears on the premises of Whole brain emulation and on the encoding assumptions behind a Memory prosthesis.

Optogenetics also underpins "all-optical" physiology, which combines an opsin for writing, a genetically encoded calcium or voltage indicator for reading, and holographic light shaping to address chosen neurons. This is the closest thing neuroscience has to a bidirectional interface with single-cell resolution, and it is available only in animals. No Brain–computer interface in a person approaches that precision on either the read or the write side. Structural mapping methods such as Connectomics give the wiring, and optogenetics tests what the wiring does.

§Human applications

Only one clinical route has produced a published human result. It treats the eye as a uniquely favourable target, being accessible, transparent, small and relatively immune-privileged. A red-shifted opsin was delivered by intravitreal AAV to the surviving retinal ganglion cells of a patient blinded by retinitis pigmentosa, and paired with goggles that convert the visual scene into amber light pulses. The patient regained the ability to locate, count and touch objects, which he could not do beforehand.³ This is a single case reported from an ongoing trial, and the recovered function is far below normal vision, comparable in magnitude to what a Retinal implants and visual prostheses delivers. Other opsin-based programmes target bipolar cells rather than ganglion cells and remain in trials. The eye is still the only tissue in which the technique has been tried in people at all.

Compared with an electronic implant, the approach puts no hardware inside the eye, and the number of addressable cells is limited by expression rather than by electrode count. On the other side, it requires a functioning ganglion cell layer, and the opsin's light sensitivity is so far below that of natural photoreceptors that external amplification is mandatory. Expressing a microbial protein in a human tissue for life also raises an immunological question that no long-term data yet answers.

Outside the eye, proposals are at an earlier stage. An optical Cochlear implant using opsin-expressing spiral ganglion neurons would exploit light's poor spread in tissue to deliver more independent channels than electrical stimulation allows, and the concept has been demonstrated in rodents. Cardiac optogenetics for pacing and defibrillation works in animal hearts. Neither has a human study.

A research tool, not yet a therapy

Optogenetics transformed animal neuroscience within a decade, yet after twenty years its clinical footprint is one published case report in a rare form of blindness. The mechanism is not in doubt. The obstacle is that every therapeutic use requires solving gene delivery and light delivery at the same time, in a specific tissue, with acceptable immunology.

§Delivery and physical limits

Gene delivery is the same problem faced by Somatic gene therapy generally: viral tropism, pre-existing immunity to common AAV serotypes, limited packaging capacity, and the difficulty of restricting expression to one cell type without the Cre driver lines that humans do not have. Editing tools such as CRISPR–Cas9 do not solve this, because the problem is delivery and cell-type targeting rather than the sequence change itself. Non-viral carriers such as Lipid nanoparticles do not efficiently reach neurons after systemic administration.

Light delivery is the constraint with no biological workaround. Blue light is scattered and absorbed within roughly a millimetre of brain tissue, so deep targets need an implanted fibre or LED. That brings back the invasiveness optogenetics was supposed to avoid, along with tissue heating and a foreign-body response indistinguishable from the one around a recording electrode. Red-shifted opsins gain a modest increase in depth. Upconversion nanoparticles, which convert tissue-penetrating near-infrared light into visible emission at the target, have been demonstrated in mice, but they add a second delivery problem to the first.⁴

Related approaches try to remove the light source altogether. Chemogenetic receptors activated by an inert drug provide cell-type specificity with a resolution of minutes to hours rather than milliseconds. Sonogenetics uses mechanosensitive channels driven by ultrasound, which penetrates tissue far better than light. Magnetogenetic schemes, which claimed magnetic control of engineered channels, have been challenged on the grounds that the reported effects exceed what the physics of magnetic energy transfer to a protein permits, and the dispute remains unresolved. Tools in this area that sound plausible do not always survive scrutiny.

§Outlook

Research is moving toward all-optical control at larger scale: reading and writing hundreds of individually specified neurons in a behaving animal, which would let investigators test whether specific activity patterns are sufficient for a percept or a decision. That work bears directly on the questions raised under Neural correlates of consciousness, since it is the only method that can impose a precise pattern rather than merely observe one.

Clinically, near-term expectations are narrow: retinal indications where the anatomy cooperates, and possibly the inner ear. A general-purpose optogenetic alternative to implanted electrodes would need gene delivery to defined deep populations and a light source that does not need surgery, and no current approach supplies both. Twenty years in, it is still an open question whether the technique's great value in animal research will ever turn into a broad clinical technology. The more likely path may be that optogenetics remains the instrument that tells other technologies, from Neuroprosthetics to Neural decoding, what to aim at.

§See also

• Deep brain stimulation
• Retinal implants and visual prostheses
• AAV vectors
• Neuroprosthetics
• Connectomics
• Memory prosthesis
• Somatic gene therapy
• Non-invasive neuromodulation

§References