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Genetically altered, laser controlled insects... Sometimes the truth is stranger than fiction. |
| Michael Schirber LiveScience Staff Writer LiveScience.com Like a hypnotist who gets a man to act like a chicken when he hears a code word, scientists have genetically modified fruit flies to jump or beat their wings when flashed with lasers. "This is a new approach to neuroscience," said Gero Miesenbock from the Yale University School of Medicine. "We can not only passively observe but actively control behavior." The remote control system was announced Thursday. It could one day replace the surgically inserted electrodes that scientists currently use to study neuron activity in the brain. "If we had impaled the flies with electrodes, we would not have been able to see the full range of their behavior," Miesenbock told LiveScience. Besides being unwieldy to work with, electrodes can inadvertently stimulate nearby neurons. The new phototrigger technique can target just one type of neuron to activate, using a genetic pre-selection trick. Lock, key, trigger The remote control setup - developed by Miesenbock and Susana Lima, both from Yale University School of Medicine - can be broken down into three components: a lock, a key, and a trigger. The lock is an ion channel - a kind of protein that allows charged particles to pass through a cell membrane. The researchers genetically altered particular neurons to have an ion channel not normally found in fruit flies. The key is a molecule called ATP. By binding to the ion channel, ATP makes the neuron fire. Typically, ATP is a form of fuel, or "energy currency," inside cells, "but there is very little of it flowing in between cells," Miesenbock said. So the scientists had to inject ATP into the fly brains. To regulate the firing of the altered neurons, the researchers isolated the injected ATP in a molecular cage that breaks open when struck with an ultraviolet laser beam. Lima and Miesenbock placed their ion channel lock in the giant fiber system, a small set of nerve cells that controls the fruit fly's escape movements -- like jumping and wing flapping. When flashed with a 200-millisecond laser trigger, flies outfitted with locks and keys responded between 60 and 80 percent of the time with the expected escape behavior. And this was not because the laser scared the flies. In fact, blind flies reacted in the same way. The laser light penetrates the flies' cuticle, or "skin," to free the caged ATP. The findings will be published in the 8 April issue of the journal Cell. A genetic switch Being able to precisely select classes of neurons to stimulate provides a separate genetic tool for understanding how the brain controls behavior. "The current way to do this is to destroy the function genetically and then look for behavioral deficits," Miesenbock said. Waiting for something not to happen takes longer and is more ambiguous than turning on a stimulus and immediately seeing the behavior you are looking for. One of the drawbacks of remote control, however, is that injecting the caged ATP into the brain is laborious. The scientists tried feeding the flies ATP, but it did not reach the brain. "Nevertheless, these constraints are really quite minimal for this clever new technique that offers so much potential for defining the neural circuits that can drive behavior upon activation," wrote Ronald Davis of Baylor College in an accompanying commentary. |
| Grant Number: 1R01DA017297-01 PI Name: MIESENBOCK, GERO PI Email: ira.mellman@yale.edu PI Title: ASSOCIATE PROFESSOR Project Title: Genetically Encoded Phototriggers of Neuronal Activity Abstract: DESCRIPTION (provided by applicant): Heterologous proteins capable of transducing optical stimuli into electrical signals can be used to control the function of excitable cells in intact tissues or organisms. Restricted genetically to circumscribed populations of cellular targets, these selectively addressable sources of depolarizing current can be used to supply distributed inputs to neural circuits, elucidate functional synaptic connections, probe the response characteristics of circuits and systems, and unveil behaviorally relevant information carried in distributed neural representations. To achieve genetically localized photostimulation, we express ligand-gated ion channels in neurons that normally lack them, and render the agonists that gate the conductances of these channels biologically inert by chemical modification with photoremovable blocking groups ('cages'). A key-and-lock mechanism thus ensures temporal control and cell-type specificity of photostimulation: the initiation of an action potential requires a light pulse that liberates free agonist (the 'key'), and a target neuron that has been genetically programmed to express the cognate ligand-gated ion channel (the 'lock'). The Objective of this project is to advance the development of highly selective phototriggers with fast kinetics (Specific Aims 1 and 2), create modular mammalian expression systems (Specific Aims 3 and 4), and initiate optical analyses of functional neural circuits in the mammalian brain, with an emphasis on networks of inhibitory (GABAergic) interneurons in the cortex and hippocampus (Specific Aim 5). GABAergic circuits are of basic importance to the processing, storage, and retrieval of information, as illustrated by the effects of agents such as ethanol and benzodiazepines, and by the involvement of interneurons in diseases such as schizophrenia and Alzheimer's disease. Thesaurus Terms: biological signal transduction, brain mapping, electrophysiology, neurogenetics, photostimulus biotechnology, cell cell interaction, cerebral cortex, gamma aminobutyrate, hippocampus, interneuron, membrane potential, neural information processing, neurotransmitter agonist, transfection /expression vector laboratory mouse To illuminate circuit mechanisms, we study explants of mouse brains in which specific classes of neurons have been programmed genetically to be light-addressable. This allows us to feed synthetic ‘test patterns’ into the circuitry and trace the transformations of these patterns in optical or electrophysiological recordings, with the intent of revealing the underlying information-processing architectures and computational principles. To relate circuit states to behavior, we work with another genetically tractable model organism, the fruit fly. We observe or induce changes in the physiological states of genetically defined groups of neurons in the intact fly brain and correlate them with behavioral states to decipher the neural signals used to represent ‘content’. |
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