BERKELEY, CA --
Carolyn Bertozzi and her colleagues at the Ernest Orlando Lawrence Berkeley
National Laboratory have found a way to use natural biological processes to
plant artificial markers on the surfaces of living cells.
With these markers, cell surfaces can be engineered to control cell
adhesion to synthetic organic polymers, metals, ceramics, and other materials
used in the walls of bioreactors, and in biomedical implants such as pacemakers
and artificial organs. In the future, living cells attached to electronic
devices may warn of dangerous chemical or biological toxins in the environment.
Already Bertozzi's group has used this cell-surface engineering to turn cancer
cells into bright targets for diagnostic probes and cell-killing toxins.
"Our primary goal is to take control of the cell surface," says Bertozzi,
who is a member of Berkeley Lab's Biomolecular Materials Program in the
Materials Sciences Division, as well as an assistant professor of chemistry at
the University of California at Berkeley. "We have begun to understand the
bio-organic chemistry of cells well enough to treat cells like complex machines
-- to really do cellular engineering."
All cell surfaces are decorated with oligosaccharides -- complex
structures strung together inside the cell from a few simple sugars. Different
kinds of cells display different oligosaccharides, and even the same kinds of
cells display different patterns depending on their stage of development or
environment. Since each oligosaccharide is chemically unique, each imparts to
the cell a unique surface for interaction with the outside world.
"We asked ourselves, how can we exploit these differences?" says Bertozzi.
Working with graduate student Lara Mahal and postdoctoral fellow Kevin Yarema,
Bertozzi set out to design new cell surfaces that could stick to synthetic
materials. "We decided to appropriate the cell's natural metabolic machinery
for assembling tailor-made oligosaccharides."
Bertozzi reasoned that if a properly designed synthetic sugar with novel
chemical properties could be ingested by the cell, the sugar might be
incorporated in an oligosaccharide and delivered to the surface. The result
would be a cell with new surface properties.
To demonstrate the technique, she and her colleagues chose an analogue of
sialic acid, a sugar which in its natural form is often found in the
cell-surface oligosaccharides of human cells. "We planned to use an unnatural
sugar related to sialic acid, one that carries an unnatural functional group.
We hoped that if the cells ate the unnatural sugar -- without noticing, so to
speak -- they would install it along with its functional group in
oligosaccharides, and thus decorate themselves with these unnatural markers."
To tag the sialic acid, Bertozzi's team needed a functional group that
wasn't normally found on cell surfaces but wasn't harmful either, one that
could react with other groups on synthetic materials -- and under physiological
conditions, such as a watery environment and mammalian body temperature.
They chose the ketone group. Rarely found on cell surfaces, ketones react
strongly with a functional group called the hydrazide; the special reactivity
of the ketone could allow a selective affinity for materials that had been
outfitted with the hydrazide group, such as ceramics, organic thin films, and
The natural chemical precursor of sialic acid is called N-acetyl
mannosamine -- more conveniently known as ManNAc -- but Bertozzi and her
colleagues fed cultured cells an artificially synthesized precursor known as
ManLev, identical except that it contains a ketone group. The cells
consequently manufactured sialic-acid oligosaccharides with ketones and
expressed them in copious amounts on their surfaces -- over a million copies on
the surfaces of most cells. Moreover, the researchers found they could
precisely control the degree of ketone labeling by adjusting the relative
amounts of natural (ManNAc) and unnatural (ManLev) precursors fed to the cells.
Cell surfaces modified for specific reactions hold great promise in the
construction of biocompatible materials and artificial organs, but the
interests of Bertozzi's cell-surface group don't stop there. "We're
investigating the cells of other organisms, such as plants and microbes. We're
looking into biosensors, in which cells designed to lock onto specific
compounds can be combined with an electronic, transducing substrate to signal
changes in the environment" -- a sort of cyborg canary-in-a-coal-mine.
Another possible use of reactive chemical groups on cells emerged while
Bertozzi, Mahal, and Yarema were developing the technology. "Many human cancer
cells, including colon, breast, and prostate cancers and certain leukemias,
have aberrant patterns of oligosaccharides," says Bertozzi. "For one thing,
they show extremely high levels of sialic acid. The possibilities were
Bertozzi and her colleagues showed that ketone-labeled cancer cells,
otherwise robust, could be made uniquely vulnerable to a derivative of the
natural plant toxin ricin. The ricin analog, synthetically armed with the
reactive hydrazide group, sought out and reacted with the ketone-labeled cells.
"It worked," says Bertozzi. "We killed 'em."
Bertozzi's group is currently moving studies of hydrazide-labeled toxins
from the test tube into laboratory animals. Other labeled cancer-killers are
being explored, as well as a method for making ketone-labeled cancer cells
stand out in magnetic-resonance imaging by using hydrazide-labeled compounds
for high contrast.
Bertozzi's team has thus become the first research group to install
specific functional groups on the cell surface through metabolic mechanisms;
the tools they used were "an equal combination of cell biology and synthetic
organic chemistry," says Bertozzi, who intends her cell-engineering methods to
be simple and rational enough to be understood and used by biologists and
chemists working together.
Bertozzi, Mahal, and Yarema have recently written about cell surface
engineering in Science, 16 May 1997, vol 276, page 1125, and in the
June, 1997 issue of Chemistry & Biology, volume 4, page 415.
The Berkeley Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified scientific research
and is managed by the University of California.