Artificial Cell Membranes Hold Promise for Medical Use
December 5, 1997
By Paul Preuss, paul_preuss@lbl.gov
Nagy's work with artificial membranes began with flat films on solid supports
when, with Mark Bednarski and Deborah Charych and other colleagues in the Lab's
Materials Sciences Division, the researchers found that certain amphiphilic
monomers (molecules with water-seeking and water-avoiding ends) form thin films
of lipids which link together under exposure to ultraviolet light; the
resulting thin-film polymer, polydiacetylene, has a deep blue color.
Sugars can be attached to the lipids to form glycolipids; the thin-film polymer
becomes decorated with randomly spaced carbohydrates. Tailored glycolipids
eagerly bind to specific proteins, including the coat proteins of influenza
virus--and in the process the films turn pink, making for a quick diagnostic
test of the presence of the disease agent.
Says Nagy, "After a year of working with glycolipid films, we found we could
form nanosized glycoliposomes (particles measured in billionths of a meter)
which change color in solution, not just on a solid support. And the color
change is much more intense." In the case of influenza, "the viruses are really
adhering tightly, as if we'd fooled them into thinking our liposomes were real
cells."
Natural sialic acids on cell surfaces can be degraded by a flu-virus enzyme;
the degradation process seems to be important in allowing flu-virus replicants
to escape infected cells. But the carbohydrate used in the flu-detecting
artificial liposomes is a form of sialic acid which cannot be easily degraded.
Thus it may be possible to develop a therapeutic use for these liposomes--by
incorporating them in nasal sprays, for example, which could soak up the virus
and stop the spread of the infection.
Nagy started thinking about what other biological systems might be fooled by
artificial cell membranes. Of several promising avenues of research, he chose
to work first on anti-inflammatory agents.
When triggered by injury or infection, the delicate lining of blood vessels
produces receptors for specific sugars found on the membranes of some white
blood cells, called neutrophils, which rush to sites of infection and injury.
Although essential to the body's defenses, excess white blood cells in the
wrong place--or adhering to tissue at the wrong time--are implicated in
rheumatoid arthritis, septic shock, and injuries that can follow clamping of
blood vessels during surgery. A method of controlling neutrophil adhesion to
the blood-vessel lining, the epithelium, could alleviate pain and tissue
destruction--and in some cases save lives.
Even when not clustered in a matrix, some sugars bind weakly to epithelial
receptors and thus partially interfere with neutrophil binding--but the same
sugars incorporated into the polymerized membranes of artificial liposomes
promise to be far more potent at preventing neutrophils from attaching to blood
vessel walls. As the "matrix" lipid of the new membranes, Nagy chose
10,12-pentacosadiynoic acid, more conveniently known as PDA.
Working with other scientists, Nagy coated glass tubes with the natural
receptor, then rolled white blood cells down this "model blood vessel" while
gradually introducing glycoliposomes. "The more liposomes we add, the faster
the neutrophils roll, indicating that fewer and fewer can find a receptor that
isn't already blocked."
"We call it the Velcro effect," says Nagy. "It's a mat of interconnections,
thousands of individual sugars on the liposomes binding to thousands of
epithelial receptors in a concentrated patch."
"It takes very little of the scientist's time to make a highly active assembly
of glycoliposomes," Nagy explains, "because we don't have to engineer every
step. The membranes assemble themselves--all we do is decide what ratios of
components to add, form them into balls, and shine ultraviolet light on them.
On each liposome, we are giving the target receptors a buffet of sugars and
other chemicals to choose from; the receptors themselves recognize the
important ligands. This approach is less than rational, more like evolution,
but it gets precise results."
Very recent, preliminary results of in vivo tests with laboratory animals
indicate that sugar-surfaced, polymerized artificial liposomes have the
potential to be powerful anti-inflammation agents. "The results are exciting,"
says Nagy, "and put Berkeley Lab in an excellent position to work with
pharmaceutical companies to develop this technology."
The "Velcro effect" with glycoliposomes may find use in other therapies: to
block allergic responses in mucus membranes, for example, or to retard cancer
metastasis. Artificial structures that can mimic behaviors of living organisms,
long a chemist's dream, are making their mark in the real world.
Nagy's artificial liposomes are the subject of a current patent application;
earlier in vitro results are detailed in the Oct. 19, 1997 issue of the Journal
of Medicinal Chemistry.
Artificial cell membranes designed at Berkeley Lab have performed
a variety of beneficial tricks in vitro, including detecting flu virus and
blocking blood-vessel receptors that can trigger dangerous and painful
swelling. Recently these lab-created cell membranes have moved out of the test
tube and into their first in vivo test, where they have demonstrated
significant blockage of inflammation in mice.
Jon Nagy of the Life Sciences Division and his colleagues have built sets of
self-assembling membranes that form spherical liposomes--shapes that may
resemble the primitive precursors of living cells.
Jon Nagy with a computer model of an artificial liposome. Some glycoliposomes detect flu virus. Others can block inflammation.
Photo by Roy Kaltschmidt
