Biological systems are inherently complex, but the human brain perhaps outdoes them all. The brain is made of billions of neurons of every flavor, wired into intricate networks, firing on a millisecond timescale and influenced by a milieu of neurotransmitters and biochemical signals—small wonder then that we have little idea how this finely-controlled hive of activity translates into thought, emotion and behavior, or what exactly is different about the brains of patients with psychiatric disorders such as schizophrenia and depression.
The rapidly developing field of optogenetics, however, is providing scientists with tools that may finally be sophisticated enough to characterize this complexity. Optogenetic techniques let researchers control specific cell types using light and genetically-encoded, light-responsive proteins. With light, the manipulation is fast, spatially focused, and minimally invasive, a vast improvement from older techniques that used electrical stimulation or chemicals to perturb cells.
“The power of optogenetics is that it allows us to unravel the cellular complexity of the brain. There are hundreds, or even thousands, of different types of neurons and this makes it difficult to tease apart the function of each individual type of neuron,” explained Professor George Augustine, an optogenetics researcher at the Synaptic Mechanisms and Circuits Laboratory at Nanyang Technological University, Singapore.
“Optogenetics nicely side-steps this problem by taking advantage of molecular genetic tricks to target expression of optogenetic probes to specific types of neurons. In this way, we can study the function of one genetically-defined type of neuron at a time,” he told Asian Scientist Magazine.
From bacteria to brains
Discoveries in a field far removed from neuroscience would provide the basis for light-activated control. In 1971, researchers studying light-responsive bacteria identified bacteriorhodopsin, a light-activated ion pump that directly regulated ion flow across cell membranes.
Getting light-activated proteins to function in mammalian cells, however, was for a long time not attempted because it was considered almost impossible. One major question mark was whether or not mammalian cells contained all-trans retinal, a chemical co-factor required for opsins to function. Fortuitously, it turned out that all-trans retinal is naturally present in vertebrate tissue, and in a 2005 report, scientists led by Professor Karl Deisseroth at Stanford University introduced the light-activated protein channelrhodopsin into mammalian neurons, showing that it alone made them fire in response to light. Other reports of optogenetic control in intact mammalian brain tissue and in live animal models soon followed over the next few years.
The optogenetic toolbox continues to grow as researchers characterize existing opsins in more detail and isolate new ones from bacteria and algae. Used in combination, opsins activated by different colors of light can be a valuable research tool. Putting one set of neurons under the control of blue light and another under the control of yellow light, for example, allows scientists to understand how each set affects behavior in a living animal.
Equally powerful is the ability to switch off neurons and to observe the effect of this on behavior. This can be done with halorhodopsin, which hyperpolarizes cells in response to light, effectively silencing them. Researchers have also engineered mutations into opsin molecules to suit certain applications; making them work faster to keep pace with the brain’s processes, for example, or more slowly so that neurons are better able to respond to other stimuli.
Professor Hiromu Yawo, an optogenetics researcher at the Division of Neuroscience at Tohoku University, Japan, told Asian Scientist Magazine that his lab has engineered a rat that senses blue light as a touch to the skin, by introducing channelrhodopsin into neurons that activate mechano (touch) receptors. The group is now investigating how different spatiotemporal ‘touch’ patterns are represented in the brain, a task much more easily accomplished with light than with mechanical stimuli.
Applications in neurology
Optogenetics has now been used to investigate the neurological underpinnings of disorders such as depression, aggression, Parkinson’s disease, schizophrenia and narcolepsy, and often presents scientists with a more nuanced picture than previously imagined. For example, researchers at Stanford University and the Mount Sinai School of Medicine found that activating the same set of neurons could make mice more or less depressed depending on the type of stress the animals had been exposed to prior to the experiment. This highlights the complicated nature of depression, and may explain why antidepressants work for some patients but not for others.
A major challenge for the field is that brain tissue scatters visible light, such that it does not penetrate very far below the surface. “This means that invasive devices such as optical fibers must be inserted into the brain if we wish to study the function of neurons that are one millimeter or more beneath the brain surface,” said Prof. Augustine.
Light that borders on infrared wavelengths, on the other hand, penetrates tissues more easily and is more sharply focused. “If we could have optogenetic molecules (both actuators and sensors) that absorb near-infrared, we could manipulate neural activity from outside of the brain and record their activities also from outside,” said Prof. Yawo.
Optogenetics is also rapidly finding applications outside of neuroscience. Many other cell types such as cardiac, muscle and pancreatic cells can be put under optogenetic control, offering researchers less invasive means of manipulation. At an even more micro level, researchers have used light to activate gene transcription and intracellular signaling cascades.
From its humble beginnings in bacteria and algae, optogenetics could someday drive a revolution in healthcare, by shining a light on the complex mysteries of the brain.
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