Targeting genes with viruses to select populations of nerve cells

Yale scientists have discovered a new way of illuminating MCH neurons, which may play an important role in regulating appetite and body weight, by using a virus that has been genetically engineered so that it cannot replicate.

MCH neurons are located in the hypothalamus, a homeostatic regulatory center of the brain. Because these nerve cells look like any other brain cell, it has been difficult to study their cellular behavior previously.

The researchers took the “safe” virus, known as an adeno-associated virus, and injected it into the brain as a gene shuttle vector, which then triggers the expression of a jellyfish gene that glows green in the MCH neurons.

The principal investigator, Anthony van den Pol, professor of neurosurgery at Yale School of Medicine, said tracking the virus in the brain makes it possible to observe what viruses do best — go into target cells and initiate gene expression.

“By creating viruses unable to follow their normal replication agenda, we can then harness the virus as an important research tool,” van den Pol said. “Viruses with altered genetic codes also have substantial value for the potential treatment of a number of neurological diseases where a gene could be selectively targeted to one defective cell type.”

He said the gene could be one that codes for a protein that enhances neuron survival, that opens or closes an ion channel, conscripts the nerve cell to synthesize a new neurotransmitter, or generates a toxin selectively in a brain tumor.

Van den Pol and his colleagues first exchanged a viral gene promoter for a neurotransmitter-selective promoter in the virus so that although the virus may infect many cells, it only turns cells green if the cells make MCH. The scientists then used thin glass pipettes to record the electrophysiological characteristics of these rare nerve cells, finding them by their green glow.

Van den Pol said scientists have struggled to identify what particular cell type is being examined within the brain because the brain consists of hundreds of cell types within millions of cells. Transgenic mice can be generated that express a reporter gene in restricted subsets of neurons, allowing recognition of live cells, but the virus approach may be simpler, faster and less costly, he said.

“When these adeno-associated viruses are injected into the brain, they initiate expression of a novel gene that continues for over a year without doing any detectable damage to the brain,” he said.

Co-authors included Prabhat Ghosh and Claudio Acuna of Yale and Reed Clark from Ohio State University.

Citation: Neuron, Vol. 42: 635-652 (May 27, 2004)