Optogenetics: Shining Light On the Mysteries Of the Brain

OPTOGENETICS: SHINING LIGHT ON THE MYSTERIES OF THE BRAIN

Author's note: This essay is a slightly modified version of a talk I gave in Second Life in 2011, a transcript of which is available at https://extropiadasilva.wordpress.com/2011/12/21/thinkers-lecture-2011-pondscum-scared-mice-and-the-global-brain/)

(Image from moleculargenetics.utoronto.ca)
INTRODUCTION: SOLUTIONS FROM UNEXPECTED PLACES

‘Convergent Knowledge’ refers to situations where a solution comes from a seemingly unrelated area of research. In an article he wrote for ‘Nature’ (‘The Creativity Machine’) Vernor Vinge talked about the need to ‘extend the capabilities of search engines and social networks to produce services that can bridge barriers created by technological jargon and forge links between unrelated specialities, bringing research groups with complementary problems and solutions together- even when they have not noticed the possibility of collaboration’.

With that in mind, let's take a look at a humble organism which helped make possible a truly remarkable technique for studying the brain

CHLAMYDOMONAS

(Image from Wikipedia.org)
There is a breed of pond scum that goes by the Latin name of Chlamydomonas. It is an algae that looks a bit like a tiny football with a tail. A German biologist called Peter Hegemann studied this algae in order to figure out how its molecular motor worked. It was already known that chlamydomonas was somehow powered by light. After all, exposing the algae to light caused the little tail to spin wildly, propelling it along. Hegemann and his colleagues eventually worked out that there were coiled-up protein molecules that studded the surface of the cell’s membrane. A photon hitting one of those protein molecules causes it to uncurl, and that creates a tiny pore in the membrane. Charged ions then flow across the membrane, changing its electrical properties. The membrane discharges a tiny shock, and that powers the tail. More studies determined which genes coded for these light-sensitive proteins. These genes were given the name ‘Opsins’.

EARLY TOOLS FOR STUDIYING THE BRAIN

Biophysicists and microbiologists studied opsins for reasons that had nothing to do with neuroscience. And yet, those very genes ended up being used as a key component in one of neuroscience’s most capable new technologies. The beginning of this example of convergent knowledge can be traced back to Francis Crick, the co-discoverer of the structure of DNA. In a 1979 Scientific American article (‘Thinking About The Brain’), Crick wrote about how the tools used to understand the brain were too crude. Ever since the 1940s, we have applied tiny electric currents to the brain in order to stimulate areas of it. This practice was first carried out by a neurosurgeon called Wilder Penfield, who applied miniscule electric shocks to the brains of patients undergoing surgery for eplileptic seisures. Experiments such as this helped build upon studies in microanatomy, which showed the brain can be mapped into distinct regions, each responsible for a distinct function. Through research like Penfield’s we now know that any experience you may have is associated with some specific pattern of neural activity.

(Image from electrodemayfieldclinic.com)
Parkinson’s sufferers' symptoms are managed with electrodes buried in the brain; drugs are used to treat depression. Both kinds of treatment come with side-effects because both affect many types of neurons indiscriminately, rather than target only the specific neural circuits that are the root of the problem. It is this lack of finesse that made reverse-engineering the brain so difficult. EEG and fMRI record averaged signals from oxygen consumption by millions of nerve cells. This lets us know where in the brain a particular mental task is being performed, but it cannot tell us how.

We have the capability to monitor single neurons, which obviously provides useful information, but on its own a neuron is not much use. As Blue Brain leader Henry Markram put it, “neurons are not islands. They need a group of neurons around them, and the minimum set of group of neurons around them turns out to be approximately the size of a column”. A ‘column’ is a kind of microcircuit, and it is the precise wiring and function of these microcircuits that neuroscience needs to reverse-engineer.

LIGHT THE WAY

Francis Crick speculated that light might be used to control specific circuits of the brain, because it can be delivered in precisely- controlled pulses. Achieving this required somehow making particular neurons light-sensitive. That is how Chlamydomona came into the picture or, more specifically, those opsin genes coding for light sensitivity. A psychiatrist from Stanford called Karl Deisseroth took a particular opsin called ‘channelrhodopsin’ (which was discovered by Peter Hegemann) and used it to create a counter-clockwise mouse. It was also Deisseroth who would give this technology its name: Optogenetics.

As the name suggests, ‘optogenetics’ is “the combination of genetic and optical methods to control specific events in targeted cells in living tissue”. We gain precise control thanks to techniques in genetic engineering. This involves using viruses to deliver channelrhopsin genes into cells. You can think of a virus as being like a tiny syringe that injects instructions for making more syringes into cells. Now imagine that all those genes are removed, and the channelrhopsin gene is put in the ‘syringe’ instead. Only this gene will be injected into the cell by the virus. By injecting tiny amounts of virus, it is possible to ensure that only specific areas of brain tissue will receive the new gene. This area can be as small as a cubic millimetre. It is also possible to target specific cell types in a targeted area of brain tissue. This is achieved using a ‘promoter’ which is a piece of DNA that controls whether or not a given cell type uses a gene. So, the viruses inject the chanelrhodopsin gene into all nerve cells in a cubic millimetre of brain tissue, but the promoter gene ensures it only gets ‘switched on’ in specific neurons. In all the others it is inactive.

THE LIGHT-CONTROLLED MOUSE

(Image from wired.com)
In the case of the counter-clockwise mouse, the channelrhropsin gene was cut-and-pasted into the right anterior motor cortex, which controls the left legs. Fibre optics were fed through the skull of the animal in order to direct light at the modified tissue. As soon as the light was shone, the mouse began running in circles in a counter-clockwise direction. When the light was turned off, the mouse stopped running and went back to doing whatever it was doing.

Since this demonstration in 2007, optogenetics has been improved to gain more control over brain tissue, which is enabling more understanding of how neural circuitry functions. An opsin called halorhodopsin was found that can inhibit neurons from firing. In other words, if channelrhropsin is an on switch for neurons, halorhodopsin is an off switch.

A useful aspect of opsins is that different ones react most strongly to different colours (or wavelengths) of light. For instance, channelrhodopsin reacts most strongly to light at 480 nanometres, which is blue light, Halorhodopsin reacts to yellow light. This makes it possible to turn on neurons in one specific area of the brain by shining blue light, while simultaneously shining yellow light to inhibit neurons in another area of the brain.

MONITORING NEURAL ACTIVITY

It is also possible to monitor neuronal activity. This is achieved by including what is known as ‘green fluorescent protein’ or GFP along with the opsin and the promoter. GFP causes the neurons that make up the targeted circuit to flash green, making it possible to simultaneously stimulate and record the activity of specific circuits. Moreover, by adding genes that cause neurons to flash when they turn on genes that manufacture certain neurotransmitters, we can (in the words of Andrew Hives at Howard Hughes Medical Institute) “potentially have each neurotransmitter assigned to a different colour GFP variant. Orange for glutamate, red for GABA, yellow for seratonin”.

THE FUTURE

(Image from transhumanismwr10206weebly.com)
So, thanks to optogenetics, we can begin mapping neural circuits in great detail and infer the computational and informational roles of those circuits from how they transform our signals. Because it is so precise, optogenetics may one day lead to implanted devices that target specific circuits in the brain, enabling us to understand exactly what causes certain neurological conditions and eliminate them without side effects. And we shall gain a more thorough understanding of how healthy brains actually work.

At the end of the ‘Counterclockwise Mouse’ demonstration, Karl Deissertoth made it clear that convergent knowledge had played a vital role, saying, “these microorganisms were studied for decades by people who just thought they were cool. They didn’t have a thought for neurology, much less neuroscience… (but) without that, we would not be able to do what we did”, which is as good a description of convergent knowledge as one is likely to find.