Bacterial genes boost current in human cells

The researchers also noted that it might also one day prove useful for treating a variety of genetic diseases involving poor conductivity in human sodium and calcium channels.

The results of this Duke University research appeared online in Nature Communications on Oct. 18.

One of the interesting features, they note, about the genes controlling sodium ion channels responsible for a cell’s electrical activity in that, in mammals, they are “surprisingly large.” In fact, their size means they cannot be readily delivered to cells through a virus, which is the vehicle so often used in gene therapy techniques.

The Duke researchers got around the problem of too-large size by delivering smaller ion channels engineered from bacterial genes to primary human cells in a laboratory setting. With the replacement channels, cells that don’t normally produce electrical signals became electrically active, and cells that normally produce signals did so more strongly.

“In current medical practice, there is nothing that can be done to stably augment the electrical excitability of cells in the heart or brain,” said Nenad Bursac, professor of biomedical engineering at Duke. “There are no drugs that can efficiently do it, and any mammalian genes that might help are too large for gene therapy applications. Our technique, however, uses much smaller bacterial ion channels that proved successful in human cells in the laboratory. We’re currently testing this in live animals.”

While bacterial genes encoding sodium channels are different than their human counterparts, Bursac noted, evolution has conserved many similarities of ion channel design since multicelled animals diverged from bacteria hundreds of millions of years ago.

Hung Nguyen, a doctoral student in Bursac’s laboratory, mutated these bacterial genes so that channels they encode could become active in human cells.

As Duke University describes it, in one experiment, the researchers placed cultured cells in several parallel lines, alternating between electrically active and inactive cells. When stimulated at one end, the electrical signal traveled across the lines very slowly. The researchers then delivered three genes to the electrically inactive cells: one bacterial gene for a sodium ion channel and two supporting genes encoding a potassium channel and connexin-43, a protein that helps shuttle electrical signals between cells. When delivered to unexcitable cells taken from the skin, heart and brain, the trio of genes caused the cells to become electrically active, speeding up the electrical signals as they raced across the lines.

“You could imagine using this to alter electrically dead cardiac scar tissue after a heart attack to bridge gaps between healthy cells,” said Nguyen, who also points out that all three genes are small enough to be delivered simultaneously by a single virus.

Nguyen and Bursac also showed that the gene encoding the bacterial sodium channel could, by itself, enhance the excitability of cells that are already electrically active. In a second experiment, they delivered the sodium channel gene to cardiomyocytes—electrically active heart cells—in conditions mimicking various diseases or stressful situations, such as a heart attack.

“In those pathological conditions, these cells become electrically silent,” said Bursac. “But when we add the bacterial channel, we can keep them conducting electrical signals under more severe conditions.”

Nguyen adds that this work contributes to a growing body of research that is looking to so-called “primitive” organisms for help with our own health.

“There’s a large pool of bacterial species whose sodium channels might have slightly different electrical characteristics to draw from,” said Nguyen. “These channels can be also modified to pass calcium ions. We’re developing a framework for others to begin exploring these opportunities.”

“I think this work is really exciting,” said Bursac. “We’re basically borrowing from bacteria to eventually help humans suffering from heart or brain diseases.”

Also on the genetic front lately at Duke, there came news late September that network and gene tools used at the Duke University School of Medicine helped quickly identify a new, rare genetic disease.

A story on the Duke University website describes the situation thus: Arriving at Duke six years ago at the age of three, the youngster had mild developmental delays and physical characteristics that included a large body and large head circumference. A genetic analysis showed mutation of a specific gene, known as ASXL2, which had never been singled out as causing disease.

The youngster’s doctor, Vandana Shashi, a professor of pediatrics for the Division of Medical Genetics at Duke University School of Medicine, told his parents their son likely had a rare and yet-unidentified disease. And she promised to remain vigilant if any other cases popped up in the medical literature that might provide additional clues, continued the Duke story. After none turned up, Shashi set out to see if the mystery case might be solved, instead, using the tools of the Undiagnosed Diseases Network (UDN) at the National Institutes of Health, which links Duke and six other medical teaching sites around the country. The participating centers pool information and innovations about diseases that are so rare they often stump the broader medical community.

By connecting to other UDN research labs and an international database of genes and disease characteristics called GeneMatcher, Shashi uncovered what Duke described as a “remarkable trove” in the form of five additional children, all with the same physical features and the ASXL2 gene mutation.

“We can now definitively say this is a newly identified disease,” Shashi said. “With just one case, we could not say the gene mutation was the underlying cause. But with six cases, all with the same ASXL2 mutation, it is definitive.”

Shashi and colleagues from other UDN institutions published their findings online Sept. 29 in the American Journal of Human Genetics.

The new disease has no name as yet, but it is said to bear similarities to two other rare genetic disorders arising from related genes—one, called Bohring-Opitz syndrome, is the result of a mutation of the ASXL1 gene, while Bainbridge-Ropers syndrome is caused by a flaw in the ASXL3 gene. Both conditions are also rare, and result in similar, but more severe impairments.

The researchers don’t know yet how the ASXL2 genetic mutation arises, but Shashi said identifying the root cause of the children’s condition is a first step, and could help drive new therapies and treatment approaches.