A genetically engineered virus has been developed that can illuminate specific, difficult-to-study brain cells and could also be used to treat brain diseases.

Researchers at Yale University in New Haven, Connecticut used the virus to do 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," says Yale researcher Anthony van den Pol. "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."

Evasive target

Gene therapy is an approach to preventing or treating disease by inserting genes into a person's DNA, usually to replace or influence defective genes.

A carrier molecule called a vector—usually a disabled virus into which DNA is packaged—is used to deliver therapeutic genes to target cells.

Adeno-associated viruses have shown particular promise in laboratory studies as gene transfer vectors because they're unable to replicate by themselves.

Van den Pol and colleagues aimed to use adeno-associated viruses to target a specific type of brain cell called a melanin-concentrating hormone (MCH) neuron.

MCH neurons are located in the hypothalamus, an area of the brain that controls various body processes such as regulation of body temperature and appetite.

Because these neurons look like many other brain cells, it has been difficult to study their cellular behavior.

Better marker

For their study, Van den Pol and colleagues injected into mice an adeno-associated virus that triggers the expression of a jellyfish gene.

The virus only turned cells green if they made MCH.

The researchers recorded the physical and chemical characteristics of the glowing nerve cells.

Van den Pol says that scientists have struggled to identify what particular cell types are being examined within the brain because it consists of so many different cell types.

Transgenic mice can be generated that express marker genes in restricted subsets of neurons, allowing recognition of live cells, but the virus approach may be simpler, faster and less costly, says Van den Pol.

"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 says.

The research is reported in the journal Neuron