Live Wires: Cells Reach out and Touch Each Other with Tunneling Nanotubes

Article excerpt

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Like teenagers, cells require constant communication with their peers. Today's teens chatter endlessly over wireless networks. Cells, on the other hand, seem a bit more old-fashioned. A clandestine web of high-speed wires physically links cells like a biological Internet, scientists have discovered.

These long, filamentous fibers are called tunneling nanotubes. They lurk in lab dishes of human kidney cells, immune cells and cancer cells. The tunnels share the same tiny dimensions as the nanotubes that chemists create with carbon. But these nanotubes aren't built by scientists. Tunneling nanotubes grow with no external interference, and they seem to offer a heretofore unknown way for cells to communicate.

"These structures have been there all along. If you don't know what you're looking for, you miss many things," says Hans-Hermann Gerdes, a cell biologist at the University of Bergen in Norway who reported the first official sighting of the tiny fibers in 2004.

Since then, a flurry of work has begun to uncover the prevalence and purposes of these conduits between cells. The covert, long-range tunnels could explain developmental mysteries like how individual cells coordinate growth into complex tissues and how immune cells streamline efforts to rapidly fight off intruders. Living nanotubes can also shuttle organelles, including the energy-producing mitochondria, between cells, a study published in December found.

Such fast, direct communication lines can also ferry unwanted guests. Just as fast Internet connections spread computer viruses, tunneling nanotubes can carry dangerous cargo. Nanotubes can be commandeered by unfriendly forces to spread disease: Recent studies show that bacteria, viruses and infectious prion proteins can all move through the nanotubes for nefarious purposes.

In one recent study, Daniel Davis, an immunologist at Imperial College London, saw glowing HIV particles creeping from an infected immune cell to an uninfected cell through the taut nanotube wires. A different study published in January found that human cells build more networking nanotubes when infected with HIV, providing a scary scenario for ramped-up virus transmission. And recent data suggest that deadly prions--infectious, misfolded proteins that cause brain-wasting diseases like Creutzfeldt-Jakob in humans and mad cow in cattle--can travel through tunneling nanotubes to infect healthy brain cells.

To date, most nanotubes have been found on cells grown in lab dishes. And, critics say, cells plopped down in a dish, away from the normal biological context, probably act a little strange, like hermits who have been away from civilization for too long. Ultimately, these scientists say, whether nanotubes carry harm or good to other cells in lab experiments may be irrelevant to the body.

"This area is very new," says Walther Mothes, a cell biologist at Yale University who questions nanotubes' significance for disease. Currently, more reviews on tunneling nanotubes exist than research papers, he points out. "We need more evidence." Such evidence has come slowly. To study and evaluate the importance of nanotubes in natural systems, scientists first had to find them.

The jungle in there

Tunneling nanotubes thread among cellular debris and other kinds of cell connections, making them hard to spot--like trying to pick out a piece of floss from a tangled bird nest. Looking through a microscope at a group of cells, "a jungle of extracellular structures" meets the eyes, Gerdes says.

An even greater obstacle to finding nanotubes was their frailty. These wires are like the ultrafine, rigid glass tendrils that remain after a glassblower stretches a vase. They readily break under mechanical stresses, the kinds of jostles and bumps that take place when preparing a cell sample for the microscope. Common fixatives like formaldehyde, used for preserving cells before imaging, obliterate nanotubes. …