ALBUQUERQUE, N.M. —Cells are expected to respond defensively when an
antigen lands on a cell membrane and prepares to cause mischief.

But to activate a response, a cell must become aware of the presence
of the intruder on its membrane, just as a human first must become
aware of a mosquito on a forearm in order to slap it.

In joint experimental work, physicists at Sandia National
Laboratories and biologists at the University of New Mexico’s Cancer
Research and Treatment Center have combined unusual techniques to make
real-time movies that show exactly how a 50-nanometer-thick membrane
notifies the cell it encloses that a hostile alien presence — an
antigen — has made a landing.

And also why notification may not take place.

“We were able to characterize the motion of the receptor proteins in
the membrane in real time as they respond to the antigen,” says lead
Sandia researcher Alan Burns. “Perhaps more importantly, we learned the
cell membrane is really complicated and highly structured, rather than
fluid and unstructured, as is the prevailing notion.”

The membrane structures, which resemble holding corrals, says Burns,
do move around in the membrane. But they restrict the motion of
proteins. The response of the cell requires that the antigen receptor
proteins cluster with other proteins to commence the cellular signaling
network.

“The proteins are like Paul Revere giving a warning,” says Burns.
“When proteins bind antigens, they begin to cluster. This causes other
proteins to thrash around. That sends a message from the membrane to
the cell nucleus that something’s wrong.

“But if there are places on the membrane that are walled off and an
antigen lands there, the cell may not be notified there’s a problem. No
protein, no warning.”

UNM researchers already knew that incoming antigens were detected by
proteins present in the lipid matrix of the cell membrane. But how
exactly to determine the process?

Burns, working with his former Sandia postdoctoral student Keith
Lidke (now a UNM professor), modified a special microscope called a
total internal reflection fluorescence (TIRF) microscope, whose
laser-light output is completely contained within the microscope
coverslip. This resembles the way optical fibers transport light,
except that the TIRF does not ever release any light. But though the
light is contained, making its use seem at first an exercise in
futility because it penetrates nothing external, it generates a tiny
electrical exploratory field that extends about 100 nm into the cell,
which lies supported by the coverslip.

“When a cell settles on a piece of bare thin glass,” says Burns,
“the membrane of the cell by definition is snuggled up against the
glass and available to the radiation field.”

Enter the UNM biology team. Led by professor Diane Lidke, the team
was able to attach quantum dots of 8 nm and 11 nm respectively to two
different types of antigen receptor proteins in the membrane.

Quantum dots emit light when stimulated by an electrical field. The
color fluoresced is determined by the size of the dot. So one protein,
when stimulated by the laser’s electrical field, emitted orange light.
The other emitted red. That way researchers could keep track of the
motion of single, individual proteins and see how they interacted;
moreover, it allowed them to observe barriers to the motion.

Sensitive CCD cameras picked up and videotaped the motion of the
lit-up proteins as they reacted to the introduction of antigens to the
membrane.

“It was like using cameras to watch individual bank robbers move around as a holdup progressed,” says Burns.

The work is of interest to Sandia, a national defense lab interested
in determining the human response to bioinfectious diseases, and to
UNM’s bioscience program.

Other authors on the paper were graduate student Nicholas Andrews
and pathology professors Bridget Wilson and Janet Oliver, all with
UNM’s Cancer Research and Treatment Center.

The Sandia work was funded by its Laboratory Directed Research and
Development office. The National Institutes of Health funded the UNM
portion.

The work was originally published online the week of July 20 in the journal Nature Cell Biology.